the problem with mobs is not inherently that they exist...the problem is they are obvious timesinks/resource drains in the worst mmo ways. they don't present a challenge from a play perspective and offer nothing in the way of satisfaction when you win or lose. and the numbers don't have to be this high. try 20 ranged low level creatures with initiative vs a 10th level party in need of a short rest (maybe a random encounter). the numbers start swinging heavily towards the creatures if they are hitting and become real groaners if the party is missing.
and what makes these mobs interesting? nothing. from an immersion pov low level mobs should exist (because mechanically you can't just ignore them)..but the impact they can have heavily skews their importance. and d&d is a ttrpg not an mmo-grinding thru mobs is super lame and dull. but the math of the game makes the mob almost a necessity for a dm. again, single high cr creatures get mowed down (numbers win the day..the party as mini mob) so the dm needs mobs...
but what if high cr creatures could actually hold their own and didn't need mobs. and the players were powerful and weren't fighting mobs but instead were taking on powerful creatures in interesting fights...sort of sounds like d&d to me.
I guess we see things very differently. I don't see mobs as timesinks or resource drains, at least not any more so than fights against a single high CR creature. There are ways to easily beat mobs (AoEs, funneling) just as there are ways to easily beat single creatures (save or suck).
I once played in a campaign, where we played an elite squad of space demons invading a planet overrun by zombies, and most of the fights were against hordes (50 - 200) of zombies. I'll grant you, there were a few boring fights (IME, every campaign has a few) but the vast majority were nail biters. We were rarely certain that we could win the fight, and that made winning all the more exhilarating.
In sufficient numbers, they definitely present a challenge from a play perspective. A well coordinated legion can be deadly. Their front-line fighters engage you, while polearm wielders and archers attack from a safe distance. You can group yourselves together to limit their attacks, or attempt to penetrate their front line to decimate the softer targets in the back. Tactics matter, and may well decide how the party fares depending on the challenge posed.
The numbers will always start swinging heavily towards the enemy if the party is missing and the enemy is hitting. That's true whether you're fighting a group of soldiers or a single dragon.
As to MMO-grinding, I played Warcraft for quite a few years and typically you wanted to take on the highest level solo creature you could handle, because that's where you get the best loot and XP. Taking on lots of weak mobs was generally only desirable if you happened to be grinding for a special drop they had.
An intelligently played high-CR creature can be quite dangerous to the party. Admittedly, if the dragon throws himself against the heavily armored fighter, while ignoring the squishes in the rear who are pelting him, he's going to go down like a chump. Because the DM played him like a chump.
Two sessions ago I very nearly TPK'd a five man 5th level party with a warlock and his invisible stalker. Admittedly, not solo, but two does not a mob make. When the smoke cleared, three PCs were on the ground dying (one a single death save away from death) and the other two could have only taken one or two more hits before being downed.
You don't need mobs to challenge a party, but bounded accuracy means that if you want to, you can.