If you are starting your game with characters at 8th level, then this balance point is moot. The Wizard is now just stronger than other choices without its main "balance" feature.
They still have terrible AC and hit points and level slower. In fact, if you start at a given XP total, which only makes sense, the wizard may start out lower level. In any case, practical experience suggests people play a variety of classes for a variety of reasons. I know that in Pool of Radiance, my "all fighter/MU plus one cleric/wizard" strategy ultimately proved to be an utter failure, even when I cheated to gain extra XP and maxed out everyone's levels. I think that game had a cap of either 6th or 8th level.
It's true, around 19th level, a MU is going to have dizzying options, but the hit point disparity only gets wider, and that vorpal sword ain't gonna wield itself. In 1e, it was a rare wizard who could outfighter a fighter, even with Tenser's transformation. In 4e, that kind of balance is actually substantially worse, since you never know who is going to crowd in on your turf.
In GURPS, you can spend your points literally anywhere you think is worthwhile. I have found that players appreciate GURPS for its options, rather than decrying that some options are more powerful in some respects than others. Options is options.
Every game does it differently. In AD&D, a wizard has to brave out whatever levels, unless you start out maybe 11th level or higher when they have access to some wilder spells, they get d4s for hit points, max out at +2 hit points per hit die, and get +1 hit point only past 10th level. They have to commit a high enough Int to learn and cast their spells. Meanwhile, a fighter is free to sink a high score into Str or Con. Barring a really low hit point roll at 1st level, he's tough at virtually every level. By 12th level, he might begin to look a little like the M-U's assistant, except, well, he owns a castle and commands hundreds of men-at-arms of his own and has a sword that can pierce the hide of any monster, and three or four times as many hit points as a M-U.
