Having a flat back 4 in those days was a viable strategy of course. The difference was, that Wenger wanted his fullbacks to play like wingers and get involved in attack as much as possible. So his coaching in that respect went against the norm as far as traditional fullbacks go.I'm not sure if it was quite as revolutionary back then tbh. For example, Mourinho then came in and played Gallas at LB. So having a flat defensive back 4 was still a viable strategy.
This is not the case now. Post Dani Alves at Barcelona we've seen the likes of Carvajal, Marcelo, Trent, Robertson, Kimmich, Jordi Alba, Alaba, Sergi Roberto being permanent fixtures in the best teams in the world. It's quite an obvious step change in my opinion.