There is arguably still no better tank out there and it's hard to improve on the M1 with a new clean sheet design.
Upgrading M1 systems at overhaul still makes a lot more sense.
The M1 is a Heavy Main Battle Tank
The M10 was supposed to be a Light Tank that could be air dropped to support infantry.
The brass kept adding requirements to the system that required adding weight that made the tank too heavy for air drop.