the terminus at Euston will cost a lot of cash. so the bean counters looked to truncate the line at Old Oak Common, where connections will be available to the Elizabeth Line. What they also needed to do was ensure that making Old Oak Common a terminus would be viable. As designed, as a through station it could turn back 3 services an hour, and it's being built like that now. Redesigning and rebuilding it to cope with even half of the proposed full HS2 service will cost more than scrapping Euston.
so it's simple economics that'll see it built through to Central London.