It's probably not hard to guess that I'm a huge proponent of league pyramids, but even I can't go with your reasoning here.
Your first "unfounded" assumption is, quite frankly, highly founded, although you're missing the point that it's not just a team's net value which decreases on relegation but their revenue. In England, the Premier League has to make payments lasting a full four years to relegated teams simply to stop them going out of business instantly by virtue of losing access to the TV money they get as a PL club. Other countries don't have to go that far, but still the teams which go down suffer badly, and if they can't get back up in about 2-3 seasons then they often find themselves stuck in the lower divisions and struggling to get back up again. At the same time, since wages are generally over 50% of a club's expenditure, those same clubs don't find their costs going down at a proportionate rate since it's hard if not suicidal to try to jettison the majority of your squad in order to balance the books, not to mention it's a guarantee of not getting promoted again. Thus, the end result of relegation is financial difficulty.
Your second assumption is not so much founded as confusing to me. If your comment is that it's silly that club owners should complain at a system that could potentially wreck the value of their investment because there's several dozen clubs waiting in the wings which could all become a lot more valuable by it then my answer is "why should that make a difference? The only ones who are voting on this proposition are those with a lot to lose. Those with stuff to gain do not get a say in the matter". If your comment instead is why should they worry about the loss of value when they will get it all back upon promotion, my answer is "that's not how finance works. Sure, the club's net worth would eventually return to where it was before, but it would lose the net effect of the gradual increase in value they would have benefited from by not being relegated. Not only that, but as said in the previous paragraph - the real issue here is that they are losing out on revenue, not on net worth. Realistically, owning a top flight club is like having a bank account with 25% APR interest, and getting relegated is like someone forcing you to withdraw half of your savings from the account for several years. Sure, you get the money back that you withdrew, but what about the 25% compound interest? That would've almost doubled your savings...
As for your third assumption, you're missing several more things. Firstly, yes there are ways of compensating relegated teams financially - but those clubs which have to sacrifice their money to subsidise the relegated sides will not be happy. Secondly, though, it's not just about the money. It's about a lot of other things they will lose. The TV exposure - the league could even force the TV companies to air just as many D2 games as D1 I suppose (though the TV companies would object and lower their asking prices as a result) but they won't get the viewers for those games so what's the point. The attendances at D2 matches will tail off noticably - even in the UK where teams have built substantial loyal support going back generations, matchday attendance generally plummets about 25% after relegation. You get fewer people buying the merchandise. You get less prestigious matches. You simply don't have the trophies available to win. These are all things that work in Europe because that's how it has always been. These are not things which are easily implementable in a country where club owners are bred to have a silver spoon in their mouths.
As for your answerable questions, I'm going to say that the average value of a relegation-battling club in Europe compared to a Europa League team is somewhere around 1:3. As for the MLS teams, you're losing at a ratio somewhere more around 4:5. That's a huge difference.
I will always love promotion and relegation and I will always be suspicious and mistrusting of closed-shop leagues. That said, I simply cannot see a way that a majority of MLS franchise owners would willingly vote to implement it.