why did the price of all cards fall so much?
Cause without Alpha-tournaments you have no usecase for this cards anymore, so Alpha-Card drop a lot in price.
Cause without access to modern tournaments you have much less usecase for untamed-cards, so untamed cards drop a lot.
Cause with Rebellion-Set people realise that Chaos Leggion cards will go out in roughly one year from the modern set, so Chaos Leggion cards drop a lot.
All sets together: Cause every edition should be stronger than the edition before old sets loose usecases even in wild-format. Just think that the Alpha-Edition has a maximum of 7 Mana, Chaos Leggion was already at 15 Mana and with Rebellion we have even the first monster with 16 Mana (don't know if even more will come) and with all this high mana-matches use-case for cards with maximum 7 Mana will decrease and decrease.
Other games (like pokemon) value old editions - Splinterlands doesn't do and this hit the whole ecosystem as every investor know if he buy today Rebllion for a huge price he can sell them in 2 years only for a very cheap price - How this can be attractive ?