I don't think it is actually about population drain or anything like that. I'll try to relate this to ČR but the metaphor isn't perfect, so bear with me. The regions with the lowest GDP per capita are apparently Karlovy Vary, Ústí nad Labem and Olomouc[0]. It's not
too bad to get to either Prague or Brno from those regions' major population centres there currently, but let's say there's a plan to revitalize the republic with a new trans-country high-speed rail link[1] including a few connections to Karlovy Vary, Ústí and Olomouc. It's expensive but it's deemed to be worth it because everyone will benefit from the improved rail network. Over time some of the planned connections are cut, and eventually the project is stripped down to a nice high-speed line which takes you from Vienna through Brno to Prague in 45 minutes and onward to Berlin. That's fabulous for residents of Brno and Prague (and I guess Pardubice or wherever it stops off on the way), but where does that leave the guys in Karlovy Vary, Ústi and Olomouc? One of the main benefits was that they'd get direct improvements to their network, and that's gone. It's nice that
after they get to Prague they could hop on a Šinkansén[2] to Brno or Berlin and get there quickly, but it's not quite the same.
There's an additional point with HS2 - one of it's main benefits was to alleviate pressure on the main north/south routes, opening them up to more regional services and an additional "northern powerhouse rail" to link some of the cities in Northern England. But the HS2 project is slowly being eroded and chipped away at to the point that it is becoming solely a "get to London faster" project without some of the improvements that alleviate the pressure on these important rail corridors. Additionally the Northern Powerhouse rail (plus things like the Leeds Tram/Light Rail) looks like it's for the chop too. So the North of England is kinda getting shafted here and I feel for them.
Going a bit further north, the benefits to Scotland probably weren't huge to begin with. Going from Edinburgh-London quicker is nice for the people who do that and shaving a 45 minutes off is cool but if it was actually extended to Scotland that would've been a much bigger deal, or maybe an entirely separate high-speed corridor for the very well trafficked Glasgow-Edinburgh route.
[0] - just grabbed it from wikipedia, https://cs.wikipedia.org/wiki/Kraje_v_%C4%8Cesku
[1] - there technically is! https://www.spravazeleznic.cz/vrt/praha-brno-ostrava-a-brno-...
[2] - sorry