TP decay is correct to have. Its not fair everyone entering a battle with 3000 TP. Its just another countermeansure regarding FFXI flaws...or we would go back to SAM era where everyone comes in with 3k TP, do WS like a bitch (specially here that you're not limited to a max of 2 WS before running out of TP) and zerg the monsters.

IMO TP decay should trigger after a while not engaging tho...only 30 secs before it start decaying is indeed a bit too fast. It should or take longer to decay or have a progressive decaying, with the amount lost per tic growing over time but being very small at first 2~3 tics.