falco's shield pressure, when done at a high level, has a huge risk/reward ratio in falco's favor against marth. Shield grabbing is not a good option at all in today's metagame, and all of marth's other punishes are not very strong. The best he can do is get a WD OOS (assuming they don't waveshine towards you or shine-grab, which mango and PP both do) or a Fair OOS. Fox, Falco, Sheik, Peach, and probably even puff (due to her low duck which can maneuver around many options and mixups for falco pressure) have better OOS options, and can also hold their shields for longer than marth can.
yeah beat, no offense, but you haven't played a truely high level falco yet
I used to think marth beat falco for a while, when I used to 3 stock everyone, and then they stopped putting themselves in positions where I was able to grab them, and started killing off of combos and not messing up fundamentals. Marth is probably the 2nd strongest "punish" character in the game, after jigglypuff (but if you ban FD... his punishes become MUCH worse). I've probably jv 5 stocked over 50 times with marth actually, in various matchups, most common ones being marth ditto, marth vs fox, marth vs falco (yes i've jv 5d many falcos) and marth vs samus. Trust me I know a lot about this subject and MU, but if you went over to Lucky's house and played Mango for hours AFTER MANGO WAS FULLY WARMED UP AND NOT MESSING AROUND, you would see how it can be. And Mango doesn't even camp me at all, it's just the pressure situations Marth lacks answers for compared to most of the other top tiers. The only character that truely has it worse is C.Falcon in terms of OOS options.
Another very problematic thing that marth has against Falco but not Fox is Falco's short hop Nair is perfect for shield stabbing marth. If marth has like... idk... anything less than half shield, it will always shield stab him. None of the other characters have this problem, and it gives very little time to think or stay in shield; another thing falco can take advantage of. I think Marth does much better vs Fox than Falco by far, at top level.