I really love Timed levels - they are my favorite level type. I understand about the old people who cannot think quickly, but i have an idea about it - give an option at the beginning of the game to whether they want Timed levels or not.
However, there is no reason to remove them (except a few ones between 144 and 609). I would really be happy if they were back. Even Jelly-Time mixed levels or perhaps new mixed combos involving Timed levels.