If you just plopped old Thiem back then he'd probably be like a fox in a hen house to some extent. Nowhere near as good as Fed still, but head to head Thiem is 5-2 and of course one of those wins was against very early Thiem. Fernando Gonzalez was a similar player to Thiem and he caused Fed some trouble. And of course Thiem was a much better player than Fernando.
Thiem's power backhand would have been a real shocker back then and the slice he had going in 2020 is very underrated. I just saw a graphic on the top backhands on tour for the last 52 weeks or something and crippled Thiem was in the top 10 for backhand quality.
No way does Thiem dominate versus the field etc., but head to head and also the fact that in general play gets betterer and betterer on tour over time. Thiem would have been a bolt of lightning in 2006 and a real challenge to both Federer and Nadal in big matches.
Instead we end up with videos like this with Federistas crowing about Fed whipping up on James Blake.