If I am pressed for time, I can simply pluck out some random cr17 monster from the MM and maybe throw in a few mooks as meatshields.
I never want to feel forced into a situation where I just take some random monsters to make an adventuring day. This sounds wrong on way too many levels.
When creating adventurers, I usually try to stick to a theme. The theme can be as simple as "Kobold Lair", or require a little more work when it combines the themes "Spider" and "Crystals". But I don't like mixing random monsters together.
Am I making my life harder by it in 3E? Maybe. But just hand-waving stuff, making up a more or less random selection of monsters, or ignoring major parts of the system just are not my style. I am tempted to say I have higher standards, but I certainly don't have high standards for an RPG, since I love hack & slash and lots of combats.
Player character gets interesting and often combat related abilities at high levels. And they expect to get to use them and see them a play a lot, too. Since that are the mechanically defining abilities for them.
Sure, leading armies, political intrigue, and what-you-have, they are also necessary ingredients - but they don't utilize the mechanical aspects much, and that is important for me, and it is important for the other members of my group. We just like this stuff.
So anyone that wants to know if high level D&D will break down for him has to wonder:
How "gamist" am I - how much do I want to use my player character abilities? Do I just care about the story that is told, and am fine with avoiding combat for the most part at higher levels? D&D probably won't break down for you, even at high levels. You will not spend that much time on this stuff, anyway.
If you love running lots of high level combat, with varied (but possible thematically still fitting) opponents, utilizing all your characters mechanical abilities (in addition to your smarts), you will notice the break down. I don't think the game will ever become unplayable, but you'll spend a lot of effort for preparing it and for running it.