Prosperity is kind of tricky to use. X can't be greater than the number of cards in your deck if you don't want to deck yourself, so if an opponent happens to have more cards in their library than you it won't be possible to deck them out. You will have still drawn your deck so you'll have a way to win regardless, but your opponents now have most of their decks in hand too so they'll potentially be able to use interaction to stop whatever you do next. Since you've drawn all of your cards that means you'll lose on your next draw step and they might just get to untap and win on their turns. The best way you have to offset all of that is to have Jace, Wielder of Mysteries on the board. That way you can make sure that X is high enough that Prosperity decks everyone and you won't lose.

It will be funny if someone bounces or destroys Jace in response to you casting Prosperity though. At that point, I'm sure that your opponents would prefer to force a draw than just let you take the clean win.

I run Drown in Dreams as one of my X cost draw spells, but I also run spells that let me get cards back from my graveyard like Eternal Witness and Noxious Revival so I can squeeze a little extra value out of the card. There have been a number of times where I've targeted myself with both the card draw and mill parts of the spell because I was really digging for a combo piece and I had a card in hand that let me get something back from the graveyard. I could also use Drown in Dreams to beat all of my opponents too. With Zaxara out and a mana combo active I can draw my deck and mill out an opponent, cast Eternal Witness to get Drown in Dreams back, and then cast the spell again to force one opponent to draw out their deck while milling out my last opponent.

In terms of the flavour of each colours' mechanics:

Black doesn't care about the individuality or humanity of what it reanimates. The corpse is just a tool to use for the Black mage's goals. Black gets cheap reanimation effects because it doesn't care about bringing the person back to life, it just wants to get those bones moving. That's why a number of black's reanimation effects also make the creature a zombie when brought back from the graveyard or depict the creature being revived as a zombie in the artwork.

White's reanimation is framed either as the uplifting of the weak (characterized by generally limiting the reanimation target to things with a mana value of 2 or 3) or as some sort of grand miracle of revival that heals and gives a champion a second chance (See higher costed effects like Karmic Guide). It tries to convey a sense of faith that all will be well and people will endure.

Green generally returns cards back to the hand instead of the battlefield because it's framed more as a new member of a species taking the place of a dead one, like when a bear gets to take up the territory of another bear that has passed away. It's all about the cycle of nature continuing on. Even with cards like Reincarnation, the flavour is that one life was given up and a new one takes its place. Green doesn't care about a given individual, once a person's time is up then that's it. Their passing gives room for the growth of another.

Blue Sun's Zenith has performed well for me in my list. I use it as a secondary win condition if Thassa's OracleGC isn't an option for some reason. You can draw your deck with the Zenith and then it puts itself back as the only card in your library. You can then cast something like Gretchen Titchwillow or Spectral Sailor to draw it again. Cast the Zenith targeting an opponent to deck them out, put it back into your library, draw it again with your creature, rinse and repeat until you win.

I gave up on running cards like Exsanguinate or Villainous Wealth because they weren't that useful in the first few turns, while X cost draw spells at least helped me get closer to my combo pieces. If I did get my combo then the X cost draw spells essentially win the game for me as well, it just takes a few more steps to finish the game.

I was wondering what you usually use Whir of Invention to tutor up? I can see Midnight Clock as an option but almost all your other artifacts are mana rocks. You could run a cheaper spell like Nature's Lore instead if you are mostly using the Whir for ramping, or maybe the slot could be Blue Sun's Zenith if you want the slot to still be an X cost spell.

You can use them during combat to suddenly pump a creature, either on attack or defense. And if you are running X cost damage spells, a good instant option is generally preferable to a sorcery speed one when it comes to removal.

At the very least, it doesn't hurt to see what instant speed options are available.

You don't really have many good options for 7+ mana instants in Red or Green. Maybe Kindred Summons if you are running a bunch of elves or Desecrate Reality if you are really wanting an instant spell with a high mana value but there wasn't much else showing up that caught my eye. You are probably better off running some instant speed X cost spells like Comet Storm, Electrodominance, or Chord of Calling instead.

If you run Chord of Calling I think that you have to use actual mana to cast it if you want to trigger Raggadragga though. I don't think it will trigger if you choose for X to be 4 or more mana and then make use of convoke to cast the spell.

I'm inclined to agree with legendofa that the printing of more support cards that increase +1/+1 counters could be part of it. If you have something with a Hardened Scales type effect on the board then a Spike creature can remove a counter targeting itself to put the now increased number of counters back on itself, gradually make it bigger and bigger.

I think that another part of the issue from WotC's standpoint is that Spikes can move their counters around at instant speed. The ability to redistribute counters at instant speed as the defending player is a strong advantage that can really discourage the opponent from attacking into you in a number of circumstances. Not knowing for sure where the counters will go in conjunction with not knowing what removal the defending player has means an attacking player could be risking a complete blowout if they attack, so they may end up being a lot more hesitant. WotC isn't the biggest fan of stalled out games because they can make for boring tournament play.

The currently printed Spike creatures don't have particularly great stat lines when compared to current power standards. As useful as their ability can be, aggro decks these days have much more potent creature options at their disposal. If new spikes were to be printed, does WotC keep to the old model cost/power ratio and risk them not see much use outside of draft or do they get scaled up to current standards and risk putting a major hamper on other creature strategies? It's a tough line to navigate, so I'm not surprised that Mark Rosewater would basically say "Eh, we're not really interested in bringing them back."