Tekken did not improve after T5 (ever since harada stepped in).
The models even almost looked similar, only slight difference every game, didn't bother to change default costumes (even more emphasis to feel the same), and the gameplay doesn't change very much from game to game.

Tekken is stale after 5. From 2 to 3, 3 to 4, 4 to 5 had major changes. T5 onwards almost feel like an update.

Didn't bother to change default costumes what?

The default 1P and 2P outfits were the same since T5,in TTT2 they even removed the 2P outfits and turned them into recolored versions of the 1P outfit.

But then again neither Paul nor Heihachi's karate Gi has changed since Tekken 1

Not every character had the same outfits and TTT1 didn't have any new costumes either.

Wait a minute, Both Paul and Heihachi's outfits, and I mean both outfits, have been different in T1, T2, T3 and T4. Different, but keeping big resemblances. From T5 onward Harada decided to keep them, in general terms, exactly the same, and I can understand that, because of the customizations.

You know,when i said both of their karate Gi,I mean their 1p outfit,you know,the ripped up looking pants and the worn out looking sleeveless top(Heihachi's with a tiger in the back and paul's with a kanji).If i am not wrong,those outfits have been there since T1

You are wrong. Those outfits are there, kind of, but with changes on every Tekken up to T4 (not counting Tag, of course). See, for example, Paul's raggy pants became raggy in T3, and he got his emblem in T2. Hehachi has different bandages, tiger is absent in T1, belt is black and so on.

P1 outfit and P2 outfit is different on every Tekken from 1 to 4.

Well it could be that Paul or heihachi hasn't their changed original Gi in 25 years(I hope for their friends and opponent's sake they do wash their clothes sometimes,lol)

I didn't catch T1 and T2,but the 2p costumes were different in both T3 and T4,they were pretty cool,but harada is saying that he'll add new stuff,maybe more costumes etc.
But i suppose namco just wants us to go wild with the customization mode so that they have to do less work there at R&.

Probably

Tekkem 2....those were the days!

*Tekken

Tekken 2 and 3, those were the good ole days.