Here a picture of a classical alternative to Eurostar Here. There are plenty of things that could have an effect, drop in fuel prices, innovations in Electric cars, and increased road building to name three very quick and easy ones.The demand for Eurostar never existed in the first place, it was based on spurious growth forecasts, it wasn't just out by a little bit, it was out by a factor of 3.
HS2 is based on passenger growth of 2.5% per year, therefore to be out by a factor of 3 would result in passenger growth of just over 0.8% or only a tiny amount more than the population growth of the UK. It should also be noted that even the last year's worth of growth which was seen as very poor showed a growth rate of 3%.
If this low (but still higher than the growth rate used for HS2) is what we see until the opening of the full HS2 network (assuming no extra growth from anywhere, not even the opening of the first phase of HS2. We would see the level of passengers expected in 2033 in 2028 (or five years early). However, if the passenger growth rate is just 0.25% higher we'll see passenger numbers hitting this levels a further 2 years earlier.
If the railways see similar growth to what they have seen in resent years and it carries on at about 5% we could see the 2033 passenger numbers achieved in 2022 (or 11 years ahead of when they "should" be achieved).
Therefore I would suggest if anything the growth rates used for HS2 are more likely to be much lower than what will be achieved.
Yes, some of the modelling for HS1 was wrong, but that does not mean that all modelling for all rail projects is wrong.
With regards to your three things that would have an impact on the use of the railways:
Drop in fuel prices would have an impact, but perhaps not as much as some think, as often the reason for not driving is as much to do with time as it is to do with cost. Although it would reduce the demand for rail travel only until such a point as the roads are so congested that they are no longer reliable to travel on (which depending on where in the country may not be very much).
Electric cars wouldn't have much of an impact as although there are some who travel by train because it is green, they are likely to be very few and far between - and anyway a train is still likely to be better for the planet. Likewise if it was to with cost then it still falls at the lack of road capacity hurdle.
Increasing road building is politicly bad news. Although politicians want to be seen to be fixing congestion they are no longer willing to spend big money to do so because all that happens in the problem gets moved along the road a bit. That and if you think that the environmentalists are moaning about HS2 that will just pales into insignificance compared to the noise that they would make about any fairly minor new road scheme.