Thiem is 6 years younger to Novak/Murray and is the lost generation.Thiem is 7 years younger. Too old to be next gen and not old enough to be part of lost 30 and 30+ year old lost gen.
He's part of the transition gen between the lost gen and next gen. These include Thiem, Kyrgios, Pouille etc.
Next gen is Tsitsipas, Zverev, FAA, Shapo, Sinner etc.
Lack of topspin on both wings - he is in the bottom 10% of the top 100 on topspin rpm. Against the very best like Nadal, Djokovic and Thiem, he will struggle to be as consistent in a 5-set match in long rallies. I’m amazed at how consistent he is against other players considering how flat he hits.