IMHO, the key strategic flaw in HS2 is conceptually the same as for UK Domestic air travel (putting aside hub connections). It's a point to point connection, over a relatively short length (the really succesful HST's are over much longer sections - Paris to Lyon, Tokyo to Osaka, etc. and were in direct competition with frequent and busy air services).
Current UK rail infrastructure is creaking at the seams capacity wise, and despite all its faults, has enjoyed significant usage growth. The real need is not to get from Euston to Birmingham faster, but to get from say Mitcham to Sutton Coldfield conveniently (ie frequently, reliably, cost-effectively). That means more trains which link together (and with other modes) to provide seamless comfortable travel. It means more Thameslinks and similar. It could mean building more capacity north of london, but that could be achieved by using much of the old trackbed of the Great Central, or expanding the loading gauge to take double deck trains on one or two northbound routes, and lengthening platforms (Uk Intercity trains are ridiculously short cf to Europe).
That is fundamentally what the Swiss have done (and are still doing), and why the new line they built is only 125mph - but they have manged to get the ERTC to work properly, so can stuff trains down that pipeline at very high frequency. however the prime motivation for building that line was to get more capacity, and get the Zurich Bern time down to just under an hour, not as fast as possible.