If you are a 21st caster and become a lich (+4) then a demilich (+6) ... you are suddenly knocked down to a 11st level caster that just happens to have great abilities, but are robbed of much of your spellcasting.
Thus, any epic party can easily gang up on you and smash your skull to tiny pieces >.>
Especially if you consider 21st level PCs are never a CR 21 challenge, in reality.
Also, good luck buying off LA 10: must reach 30th level before you can start to buy them off.
It is either LA 4 or 12, never 10, and there are several ways of increasing one's CL (exs: Orange Ioun Stone/Archmage's Arcane Power/Magic Tatoo). You also forgot you do not get de-leveled upon becoming a demilich; your ECL breaks through the roof and future leveling becomes harder, bit you do not lose the progress you had made so far, hence, a 25st lvl demilich is, in fact, a 25st level caster (CR 33/ECL 37) with great abilities and a some deal of difficult leveling further. Even more importantly, you did not account for the lich template being taken as early as the 11st level, which translates to the possibility of paying off two sets of LA, rather than a single one made of their sum, greatly reducing the task's difficult.
The LA is listed as either 4 or 12 due to the lich's base 4 and an increase of 8 to a lich's ECL on the NPC example provided by the book, which conflicts with the null LA cost. While the DM can, of course, rule otherwise, the listed LA takes precedence for PCs, meaning the "vanilla" LA is 4; a 30th level demilich could have bought his LA off to 0.
Powder Miner, that is the reason campaigns rearely touch the higher tiers; combat classes can match and even out perform casters during the first two dozen levels, but when things go epic that changes, brutally so.