@moorebrother1 it is not that unusual that during the release of a new (hyped) standard set, prices of eternal cards are declining. Players getting out of their expensive staples to buy into drafting/the new standard meta. That + the EV of treasure chests and changing drop rates of those staples I think are more of a reason for the current prices then the whole arena debate (the price of standard on mtgo isn't that low at all).
collections on mtgo will probably decline in the mid to long term due to arena, but before that i think we might see rising prices on those staples again, for example when modern is the talk of the town again in spring. (excuse my bad english!)
@moorebrother1 on Karn specifically:
when just considering the "monetary value", I would wait on buying. the set hast just been released, normally most of the cards are getting cheaper over time, because the new set gets drafted a lot (hence the cards entering the "MTGO system"). Karn will maintain its current price tag or get even more expensive if he is getting played in standard or modern a lot (cause that's mainly what most people playing online, sometimes pauper and legacy can affect the prize as well, the latter especially with the team pt coming up). Otherwise, he most likely will decline over time (to a 7-17tix range maybe?!).
On the topic of "buying into MTGO" in general:
the value of mtgo digital cards isn't just a plain downward trajectory. Even if one isn't going 5-0 in leagues consistently or win challenges etc., the trading system of MTGO makes it possible to at least in some way "grind yourself" to a sizeable collection, even with the cupple of tix you get by creating a new account. (that and the existence of third party bots is probably why they are using a different economy model within MTGA)
I think this relates to what @Chronatog is asking, and I am neither from the US nor a lawyer so I don't know (and maybe wotc adresses the gains aspect in their terms and conditions?!), but there are some patterns to be observed in the online economy as a whole as well as in specific cards.
For example just as right now every new standard set release will cause a (slight and often just temporary) drop of prices for i.e. some modern staples. A same temporary decrease can be observed with reprints of staples within Masters Sets. There are articles (i.e. over on mtggoldfish) that are explaining those patterns more elaborately and in depth than I am able to.
Moreover and other than in paper (where it is much much slower and more lineal), the value of those staples is of a rather fluctuating nature, with some seemingly having some kind of "invisible" bottom and top as well as on different amplitudes.
There are and have been even more of what some may call opportunities, from using arbitrage between bots to reacting quickly to real life implications (spoilers, cards on camera during pt coverage, you name it) - in some ways, the MTGO economy can be perceived as a stock market on speed sometimes. On a lower scale (since one can't just buy millions of one card), but with much larger percentual changes and maybe even more of a "safety net" since it is such a small ecosystem with just so many things that can influence value.
I don't know if it is legal, but it is at least possible.
Hope that gives at least some perspective to the financial aspect of the topic. Plus: excuse my bad writing since I am no mother-tongue speaker.