In the case of Europe, a lot of older unelectrified mainlines were just replaced by HSR along the exact same route. The HSR lines were to be not just a simple electrification job, but an actual upgrade to the line as a whole.
Though, in my opinion, it makes a lot of sense for a lot of places to electrify anyway for the sake of commuter or regional rail. Much in the way the likes of Septa, Metra and MTA have electrified commuter rail corridors, it would make sense for a lot of these sorts of lines to get electrified and get upgrades like these in parallel to any HSR construction.
Point to point HSR doesn't really work that well. The solution is that of providing connections at both ends, or else you just end up with people who think "If I have to drive the other end, I might as well drive my car the whole way". Along the NEC, Boston, NYC, Philadelphia, Baltimore and DC all have extensive regional rail connections which allows for onward journeys by other trains.
Meanwhile, Brightline/Virgin US is a connection from Miami, which does have a metro system to West Palm Beach, which has absolutely nothing other than maybe buses and taxis, and I'm not even sure about that.
Meanwhile XpressWest only fixed its problem by extending its proposed line to connect to CAHSR. Previously, it would have terminated in Victorville from Las Vegas. Victorville only sees one Amtrack train a day in each direction, while the Las Vegas terminus would have been connected to the Las Vegas Monorail. While the extension means that this is no longer a useless HSR project, the issue America has is a lack of smaller, slower speed regional rail projects in parallel. I think politicians in the US are too caught up in autonomous cars and Elon Musk's "Loop".