Youre concerned about spell density in a hypothetical very long game of Magic
They lasted until the beginning of the Diamond/Pearl era, so they were available from September 2004 to September 2009
If a cards value is driven by utility rather than collectibility, the price may drop quickly if it is replaced in the meta or if tournament rules change
Finally, the release of Paldean Fates will likely not change the balance between decks very strongly, but as I mentioned in my preview [ Gardevoir and Lost Box are the two decks that benefit the most from new cards (Moonlit Hill and TM Crisis Punch respectively), and theyre decks that might challenge Giratinas top spot, so well have to see what happens then!
The longer a game goes on, the more Phyrexian Arena puts you in the lead over your opponents, as youll have access to more options and more resources
Time to look for more inspiration, or at least a road map for how to put this thing together