Even if you count Euston+KX+SP as a single station, it's not that much busier in trips HS2 users from Birmingham might want to make since most trains have that as a terminus going north.

Realistically, you would only be adding access to the Thameslink and to the Northern and Victoria lines.

Old Oak Common has access to the Elizabeth Line, the two branches of the North London Line Overground, and a branch of the Lionesses Line Overground. IMO increasing Overground speed and throughput might be a better use of money than the branch line to Euston.