And in this scenario, we are offering the high level spells ON TOP of a ton of abilities.

For example, a 16 HD monster class can finish out with the last 4 levels in a full caster and get ALL the ninth level spells of a Caster 20. The succubus is a 7 level class that strictly superior to Sorcerer, and you can get all the goodies of a level 20 sorcerer (except, possibly, the benefits of a max level familiar, not that it matters) plus 7d8 hit dice that come with medium bab, 8+Int skills per level, +7 cha, and a good Reflex save. That's before I even look at the actual class abilities.

Yes, it's an issue that makes many caster monsters less appetizing. No, I don't think that it would be horribly unbalancing.