Is this is a realistic discussion on combat or a discussion on realistic combat? Because it seems like the latter with the implication that M&B potentially has the best melee combat. It's not the best melee combat, that sounds like something claimed by people who simply prefer that style of combat. It is, however, relatively realistic combat and people seem to love to mix the two.
This game is aiming for a mix of Absolver, fighting games, Dark Souls, and For Honor, through the use of its stance system, switching between styles, and the use of a predefined moveset. This game is not going to be as realistic as many of those games (in terms of control over the weapon), but at the same time I'd say that I consider For Honor to be the best melee combat in gaming right now. It's not as realistic even if the techniques thrown are realistic, but in terms of fluidity, weight, room for skill, and general enjoyment factor it, in my opinion, infinitely beats out combat systems reliant on what M&B, Mordhau, and friends have successfully shown can get dull very quick. For Honor has its own share of problems, but that's independent from combat, and it shows realistic weapon control =/= best.
Freeform combat quickly loses its marvel when people can block on reaction a few days into release, at which point it just becomes tedious, reliant on who can spam feint more, and is generally not something most people would be willing to play with for any lengthy period of time - and certainly not something that most would find entertaining enough to keep them in a game that requires you stay around for literal years to attempt to see what the combat system truly allows. Part of maintaining a playerbase in a long-term game like that is room for growth, something every other profession and nuance of the game will have, and that isn't something you can get with the realistic weapon control approach unless you're willing to intentionally gimp gameplay for a couple years.
As for what we can sacrifice, could you elaborate? Are we talking about technical issues like desync, ping, etc? I'd say I wouldn't want desync, but I've also learned to adapt to it due to how many MMOs over the years have been plagued by it. Luckily, the devs have put such a heavy emphasis on "you need decent ping for this game" and serverside data that desync isn't likely to be as big an issue; that tends to come from the client interpreting where someone will be incorrectly, which in turn generally comes from one side having higher ping. With their adamant stance on the game's data usage, and even having OCE servers, there's likely to be fewer desync issues than most games. But if I have to arbitrarily choose something to sacrifice, it'd be that. Engaging gameplay itself is too important to me.
Or are we talking about the depth and nuance of the system? Because I vastly prefer what we've been promised. A ton of fighting styles, the choice to customize which styles you use, and Absolver/fighting game-esque gameplay. The team also likes to talk about player skill vs character skill and how your character can improve over time - all of those games, minus Darkfall (loosely), are pretty much the polar opposite of that intent. Those games also imply you have to fight in first person the vast majority of the time, whereas this game seems to be more third-person focused with the option for first-person.
As a person who both competitively plays fighting games and enjoys traditional MMO PvP, this game's combat drew me in as it was promised and marketed. M&B, Mordhau, and Mortal Online, while enjoyable in their own ways, don't have the depth that I care about, don't have the skill ceiling for combat that I care about, and also clash with the game's ideology in a number of ways. I don't need a hundred different movesets, but with what they've implied, it appears that's probably what we'll get - luckily, if people are worried about how much effort is required to take that many, they've also spoken about how not every single move in every single moveset will be unique. They'll reuse more than enough animations between similar sets with similar weapons, it's just that each moveset represents a different goal overall.
Finally, the claim that "sacrifices must be made somewhere", while pragmatic and helps prevent setting yourself up for failure, can't actually be sourced. I'm sure there will be some hiccups in some way come release, but I'm not going to randomly start claiming what things I'd be willing to live without because of what's basically 'they can't possibly deliver the full promise'. I want flexible gameplay, one that doesn't result in constant rubberbanding, and a meta that isn't about who can feint more (and whether you've mastered the timing to react to such with muscle-memory) or Chiv-cripple their character into stabbing their opponent at impossible angles. I want fights to be beyond "once you've seen one high level fight, you've seen them all". What they've promised so far matches all that. While they are aiming for an upper echelon game overall and intend to beat out everything else in the market, those fundamentals aren't much to ask for as plenty of games have pulled it off. There's no obligation to switch to first-person swing-and-feint gameplay to somehow remedy it, especially since that's just as prone to its own slew of issues.