You pulled that 90% out of your hat, didn't you? I can claim its 50% and be as sound as you are, if not more.
in 90 % of the cases, balancing for high level play will balance for low level play as well.
Besides, there is WvW, and it is totally different kitchen. You can have 100% balanced game in tpvp and glaring whopping imbalance in WvW at the same time.
Well, you said it yourself. Some stuff is easy to learn. Some stuff is hard to learn. Example? Elementalist. There are much fewer of them in spvp than in tpvp.
Because high level play decides the meta, and low level play follows high level play in terms of meta. Besides some stuff being easier to learn, high level balance=low level balance.
Also, some stuff is extremely good in coordinated team, and has barely any use in random team. Like mass invis / portals. Or Sanctuary. Or many, many more things.
Point being, high level coordinated play != low level random play. They are very different. Thus, the balance IS different.
Example from, say, old Dota. Old Rikimaru (with perma invis and pew-pew-pew ward as ulti). He was totally broken in random pubs. I remember pwning faces 20 kills / 0 deaths with some stupidly ridiculous builds with Sheepstick and Dagon.
When it came to clanwars, that character was never ever picked. Because people were coordinated enough to get true sight and protect the carrier. And even if Riki got close enough to place a ward, ppl either retreated from or focusfired it so it died in 0.5 seconds, being a waste. And without ward/invis he was just another melee squishy with no nukes / cc / support.
It is still a goal for ANet, you see. So we should never disregard stuff based solely on the argument "Hey thats not tpvp so it doesnt matter".
Of course balancing both perfectly would be awsome, but probably not a single game mananged to balance high level pvp, let alone both high and low level pvp.
Well... I never noticed a declain in numbers of D/D (or even S/P for that matter, but they were never too popular) thieves in tpvp. They are still whopping effective roamers. They still oneshot people with CnD->Steal->Backstab->HS combo with no telegraphing whatsoever. They still get away freely from almost any situation. New Assasin's Signet, for instance, changed nothing in this regard. Maybe some of them adjusted their traits slightly, but that's it. Many did not even use the Signet in the first place.
And saying that PW and HS nerfs didnt touch Tpvp is a stupid arguments. If anything it made thiefs even more pushed towards the venom share build, without any other options on par with venom share.