Mustrum_Ridcully
Legend
If you want 10th level spells (and spell slots), check out Arcana Evolved. 
If you'd continue the spell level progression, you indeed would end up with 10th level spells (available at 19th level for Clerics, Druids and Wizards, and at 20th level for Sorceror*). But for whatever specific reason, the D&D designers decided that 10 levels (0-9) of spells would be sufficient. I don't know if the FR story has a lot to do with it (it at least explains why there are no higher level spells).
*) Note that Bards would probably also have to go up to at least 7th, maybe 8th level spells (their progression is somewhat llike "spellevel x 3 - 1 = min. caster level required for spell level"), and Rangers and Paladins would have to go up to 5th level spells.

If you'd continue the spell level progression, you indeed would end up with 10th level spells (available at 19th level for Clerics, Druids and Wizards, and at 20th level for Sorceror*). But for whatever specific reason, the D&D designers decided that 10 levels (0-9) of spells would be sufficient. I don't know if the FR story has a lot to do with it (it at least explains why there are no higher level spells).
*) Note that Bards would probably also have to go up to at least 7th, maybe 8th level spells (their progression is somewhat llike "spellevel x 3 - 1 = min. caster level required for spell level"), and Rangers and Paladins would have to go up to 5th level spells.