I feel that there should be a way to keep playing beyond cap, and I think the TR system works perfectly. Is it grindy? Yes. Especially if you solo. If you're going for multiple past lives and want them fast, you'd better find a static group that's for sure.
But I'm against increasing PL benefits (balancing them, i.e. buffing SOME past lives would be ok tho), and against decreasing the time needed, simply because people who DON'T want to TR would be disadvantaged either way.
And 18-20 is no big deal if you consider that at 18 you can basically play any end game content except epics, especially guild raids and such. I do believe amrath and cannith quests need more xp tho.
Past lives are supposed to provide minor (yet not pointless) goodies to people that have time and will to grind the xp again and again. If you don't have the time or don't wanna grind, you can still play anything with a non-tr'd toon. I've seen a lot very good 1st life toons around, even some DC-based casters.
Although now many TR just because TR1 is actually fast and easy and definitely worth it even if you don't have much time.