I think that is exactly the core problem as far as why so many people are opposed to HS2 is concerned. (Though obviously there will always be people who have quibbles because they disagree with aspects of the route etc.)
Then there's the hardcore maglev and upstart hyperloop crowd who are joining the bandwagon calling for HS2 to be scrapped. The problem with that, even from their alleged interest point of view, is that such a course of action is likely to lead to yet another generation of lost opportunity while options are researched again, even if a new scheme based on such technologies was suitable, available quickly for implementation, and represented a good BCR (bearing in mind conventional network compatibility benefits of a rail based solution with respect to potential reach of new infrastructure and phasing benefits of an evolving network that cannot be realised with a new type of guideway).
Maybe that's something to campaign on for the future. After all, didn't the French TGV network start with just one single line? My guess is that HS2 will be very successful once it's running, and within about 10 years of its opening, the debate will be about how to turn it into a proper network of. 100-125mph+ lines. And at that point having something running down to Plymouth would seem a fairly obvious link in any highish-speed network.
While the alignment will be curved and graded for the initially specified high design speed, it seems likely that actual operational maximum speeds may be quite a bit lower to save costs. That will not only be on initial capital expense of track construction and day to day traction energy, but probably also on whole life rail and wheel regrinding and renewal expenses, which climb very steeply and non linearly with speed (That is one point on which the maglev enthusiasts are correct). It is apparent in both Europe and East Asia that maximum speed is not typically exceeding a fairly long established 320kph figure, which to me seems a sensible trade off. The latest lightweight and super-powered rolling stock can compensate for a lower maximum using it's stunning acceleration capabilities.
Take Germany for example. On much of the ICE network, 250kph max is now being specified, with new super efficient and high performance 4th generation trains. Overall, that is a sensible compromise on commercial speed versus costs, especially for the majority of routes that have fairly close, even station spacing, so little opportunity to exploit any sustained higher speed running. ICE4s will mean a small drop in maximum speed in certain places, but the ICE1 and ICE2 predecessors are much heavier with less power and tractive effort, so the new trains will be able to match and even improve on journey times. There are a small number of premier 'neubaustrecke' that have longer non-stop segments and are to retain their current 300/320kph limits using ICE3 and future Velaro or similar units, but in terms of total train km operated these represent a fairly small part of the total service offering.
On HS2 the higher than strictly neccessary design speed of the alignment will assist with wear avoidance as gentler curves in particular will save on side wear of rails and wheelsets. The alignment's higher speed spec could be seen as future proofing for as yet unforeseen developments, in much the same way as Brunel's 1840s engineering of the GWR allows todays high speeds, clearly unforeseen at the time.
As to a developing network, I think HS2 is a one off as a complete new end to end route for the UK. It represents a unique opportunity to relieve three significant main lines with one trunk. I believe future GW improvement is much more likely to consist of a number of substantial high speed cutoffs for the existing Peninsula route to speed up and segregate traffic, perhaps starting with the Hungerford area on the B&H and then maybe a Dawlish bypass. A similar approach has been suggested as an alternative to HS2, but the sheer quantity of works required, and the disruption it would cause, on all three main lines, to make an equivalent capacity difference renders that method impractical. Remember the main trunk will host 18 trains an hour. There's no way Devon and Cornwall could ever possibly support that kind of traffic.