That is an interesting view, but in a Bo3 scenario I don´t believe your opponent should be allowed their best stage at all (FD in this case, unless bad marth then just techskill till they die).
Lets first analyze something if we were to play first game in FD and lose, here there are two ways you can go about beating this guy, assuming he bans Dream Land, as most marths do:
If in the first game you see he enjoys running around, and is very centered in moving around you will probably strike to Yoshis and probably win. If so, you will then ban FoD, and probably get counterpicked to PS, which can be considered a stage towards fox, but plays right into the enemies comfort zone.
If in the first game you see he is a more static and as I call them RPS marth, you will want to outrun them, and will strike PS, assuming you win, the marth will take you to BF/Yoshis, as you banned FoD, which again plays into their comfort zone. A smaller stage to where the RPS solution is more leaned towards the bigger hitbox.
So in both occasions you actually play towards the opponents strenghts in the end, which will not happen if you win game 1 in any other scenario.
Now let´s picture the supposedly worse scenario, but before this lets ask something, how bad really is FoD for Fox vs Marth? Here is where I believe there are a lot of problems in this mu, it is true that against marth platform camping in the side platforms is null, and that he lives longer in this stage but other than that this stage is actually not that bad at all. Here are the reasons why:
-Stage is vertically long, allowing for Fox to actually live a bit longer if he was getting gimped, it also has a wall to tech with and walljump, giving you mixups around gimps.
-When stage has no platform or one platform, it still isnt fd, with proper di towards the platform and away from the fairs, (sometimes cant be both) it will always give you a 50 50 escape towards top platform.
-Stage is both long for Fox dash dance and short for a Marth that also dash dances, straight up giving you the advantage to bait him out.
-Stage being big isnt really as important compared to other mus such as peach and puff, because marth´s recovery is lineal and easy to react to, if you practice edgeguards, there should not be any marth that escapes (other than top level ones, those are harder).
Considering these, if supposedly you lose game 1 in BF and then strike to one of the other stages accordangly and win, banning FD, and Marth picking FoD, you really should still have a big advantage, I do not know why this stage is considered to be annoying for spacies, other than tradition and other floatie matchups.
In conclusion, trying to win all games is the most optimal way to beat a player, it is true that we need to feel them out for a bit to adapt to them, but we should aim for that being only 1 stock or 2, and then still winning the first game, such as most top players do.
This is all my experience and knowledge that are meant to benefit me, if you believe this analysis was the correct one, then try to expand on it and put it to play, if not, then you probably see the game differently, this game is not meant to be played the same.
Love, a guy with a cool Spanish accent.