I'm not too technical, but I know bitcoin has proof of work built in which is how mining works. Why doesn't bitcoin have proof of stake? Proof of stake seems necessary to ensures the security of a crypto network long term.

I'm reading about PPCoin which is the only coin with proof of stake built in. Ppl are saying this makes it the best alternative crypto-currency blah blah blah

If an alt coin that has proof of stake (currently only ppcoin) became popular, would bitcoin implement it?