We are making a bunch of assumptions here...
1.) Monsters will use some manner of different rules than a PC, and thus the two will be incompatible.
2.) Monsters will be hard/impossible to reverse engineer.
3.) Monsters cannot be customized without constructing a new monsters.
4.) Simpler = less options.
5.) Monsters as PCs will be long gone.
Deep breath, relax.
1.) I suspect a monster will be able to slap class levels on, at least the ones that used to "advance by character class". Really, A troll is going to have an attack bonus, three saves, an AC, a pool of hp, etc. You want a troll fighter, take the MM troll stats, give it a +1 to hit, +2 to fort saves, 1d10 hp, a new class skill, and whatever feat/maneuver choice a first level fighter will get. No worrying about LA, CR, etc. I'll assume the rules will give a guideline for adding a class to monsters to determine XP amount. Problem solved.
2.) I suspect monsters will still have some linear way to advance or to create new ones. I just don't think monster HD = NPC class levels any longer.
3.) Again, I think some monster abilities will be able to swapped out easily. Templates, "monster abilities" or such will probably have some modularity.
4.) I think monsters will have a smaller range of options, but more options as to what to do. For example, a dragon rarely uses all its sorcerer spells. It uses one or two (dispel magic is common) but it doesn't need the complexity of having a 12th level sorcerer built in. Loosing them is fine, IMHO. A Mindflayer uses some manner of charm/dominate ability, mind blast, and eats brains, it doesn't need much more than that.
5.) Many monsters in the MM now have no rules for PC advancment. Displacer beasts advance by HD, Skeletons don't even do that. The only monsters with "X as characters" are humanoids or the like (goblins, planetouched, lizardfolk) who advance by class level. In 4e, I expect those same monsters will have a "X as PC" line with some bare racial traits (which won't necessarily match up with a stat block, but definitely give a "baseline" race to use). So I suspect orcs, goblins, kobolds, gnomes, drow, and other "favorites" will get the treatment. Those who don't will have to create a new race (or extrapolate from the stat block), much like we did in older D&D. I don't think D&D needs rules for playing as a weird or exotic things, if you want that, that should be the DM's option to create something balanced and call it a "skeleton" or "draconic" race.
I think we're not losing customability, but just seeing a shift from highly numerical monsters to a more modular, fluid option.