FormerlyHemlock
Hero
Yep. And your analysis was more or less fine (though it did have an issue that you chose to ignore the beholder's anti-magic when all of the attack against it were magical).
But the issue with the example was that what works theoretically doesn't always work in practice, and while the theory says that creature lasts two rounds, that doesn't match with what I've experienced in practice. And, really, it's the practice that matters.
And one of the reasons the theory doesn't match the practice is...
Exactly this. Amongst other things, if the beholder has a bunch of allies, then those allies serve to soak up a lot of attacks, meaning that lots of that lovely DPS is 'wasted' - a 20 damage hit to a 14 damage hobgoblin 'wastes' 6 damage. Or, perhaps more to the point, it means those 20 points aren't coming off the beholder. Probably.
To be fair about this though: 5E achieves short combats primarily through making combats easy. A lone beholder is a Medium combat, over in two rounds after contact is made in earnest. "Contact in earnest" isn't necessarily synonymous with "you see a beholder" or "roll initiative". The GWM Barbarians might have to jump through some hoops to bring their melee attacks to bear on the beholder. It's impossible to estimate in general how long that will take though because it's very terrain- and DM-dependent.
The point is though that 50 hobgoblins + 1 beholder is no longer a Medium encounter. It's a triple-Deadly encounter. On the one hand, I highly recommend triple-Deadly encounters and above because unlike Medium encounters they are not usually boring and contrived. On the other hand, it's not a surprise that a triple-Deadly encounter takes much longer to run than a Medium encounter, not least because the players will be taking longer in real-time to make their decisions in this high-stakes environment.
An encounter like that may only feel like a few minutes, but it's not at all unlikely that at the end of it you look at the clock and discover that it's really been two hours.