They'd have to add a new skill, which would raise the cap by about 10 levels.
Furthermore, there are 91 perks for warriors. The cap is at about 81 or so. One (or two) complete trees for Warriors are redundant depending on your build (Block, Two Handed, and One handed. If you go dual-welding, you don't need Block or Two-Handed. If you go Sword-and-board, you don't need Two-handed. Go Two-handed, and you don't need One Handed and Block.)
Furthermore, there are 74 Stealth perks and 86 Mage perks. Mages should be the ones complaining, as none of their trees can be completely removed becuause of redundancy (though 3 perks can be ignored because they deal with bound weapons, something a mage may not invest in)