Of course, he was popular before that. He was fucking cool in Baratie and Arlong Park. I just mentioned some of his prime moments in the series. If you make a list of his best moments in the series, I bet you that the top 5 or maybe even the top 10 are all going to be Pre-TS moments.
The moment he saves Usopp from Enel and takes down Maxim, "Thanks, I needed a light" DAMN. Pre-TS Sanji >>> Post-TS Sanji any day and I'm willing to bet that most people will agree. If you think that he's been better or even as good since the TS then you're just... wrong.
Yeah, that was probably Sanji's best moment in Onigashima till now and even that ended with him thrown away by King. Not saying King is weaker than Sanji, Sanji should've been to do that or any sort of that bs. Point is Oda didn't have to do that. Could've played it out like Sanji escaped or something but no. It's not even the first time. Happened with Doflamingo, and almost happened with Vergo. You're lying to yourself if you think that Oda has been treating him the same way as he did before the TS.