I think I’d argue the opposite that most really good / successful teams have an exceptional defensive midfielder. In fact I’m struggling to think of a team who’s won a major trophy without one
Busquets, Rodri, Kante, Fabinho etc
I’m not in anyway comparing Souza to any of the above, but I think it’s an essential position in modern football and don’t think Stuart Armstrong would be a wise replacement for Souza.
If we played a flat 3 in midfield with Hamer and Armstrong either side of a defensive midfielder that could work but replacing Souza with Armstrong would get walked through imo