I'm sorry but these are just not very realistic scenarios for more than statistically insignificant cases.
People drive because it's both of convenient and reasonably priced, all in. Rail is almost never both and is often neither. That's what really needs changing. Rail had some degree of market power in 2019 and the years leading up to it, enough to start enabling that change, but look how far we got. In 2020 it has next to no commercial influence.
I agree that car use is high because, all in, is a reasonable price. However that's often because you are doing 3,000 to 9,000 miles a year for work (7 to 20 miles each way).
If you are significantly reducing this by working from home then for many the reduction in vehicle costs beyond fuel and parking charges at work are fairly small.
As such the cost per mile for the remainder of your travel increases.
Given that the average car costs over £3,000/year all in (and yes there'll be some for whom this is lower, but not everyone is able to do that, and anyway all that does is shows up that there's others who are paying more) then once you start using it for much less than 6,000 miles then the cost per mile starts to increase noticeably.
If we assume £3,300 whilst doing 10,000 miles per year then that's 33p per mile.
If that falls to £2,900 whilst doing 7,000 miles per year than its 41p per mile.
If it falls to £2,600 whilst doing 5,500 miles per year than its 47p per mile.
It should also be noted that statically 25% of miles traveled are sub 5 miles and so there's scope to reduce car use further than just to/from work.
For instance, if there's no longer a need to drive to work then there's no need to drive the kids to school. With my local schools it's possible to walk from most of the catchment area to school, drop the kids off and walk back home before 9am (Covid-19 will change this site to staggered start times, but it'll still be a 9:15 start at home at the latest).
Yes, certainly rail use wouldn't be suitable for everyone, however assuming a 10-20% gross fall that's likely to be a net fall may not be that much by the end of this year and is likely to be smarter than this by the end of next.
Next year would likely result in trains being busy, but less people standing. Now for London commuter trains this may allow some of the peak trains to be (say) 8 Vs 12, as that's where the fall will be felt the hardest, however there's often people who would use rail but don't due to there but being space, so some of those losses would be reversed anyway. However beyond that I suspect that it's mostly going to be slightly quieter trains.
If you look at some of the busiest trains (over 150% full) then a full 40% reduction would put them at over 90% full. Therefore many of those trains might actually become pleasant to travel on.
Even if into London travel is a greater fall than the average of 40%, then chances are it would allow frequency of fast and semi fast services to reduce and use some of that capacity for running trains which would otherwise be useful but there's not the capacity to run them. How often is it said it'll be useful to ... only to be told that there's not the paths to do that because of the existing trains.
A standard class coach with 66 seats which is currently full with a 15% fall in passenger numbers would then have 10 spare seats, however if it's normally got 4 people standing then it'll have 7 spare seats. Conversely it having 30 seats filled then it'll have 26 seats still filled.
Overall it'll mean that the busiest trains would be fairly OK to travel on whilst having a barely noticeable difference on the quietest trains. Even before accounting for any change in mode, with that then likely to reverse some of those falls.
Yes it works impact the profitability of the rail network, however so does many other factors (which the government are happy to cover) such as strikes over Driver Only options. However, much of what people get excited about in terms of government support for the railways is actually signing in enhancements rather than the cost of the day to day running of the network (which had typically been less than £500 million a year).
Even a 20% fall is only going back to what we had 5-10 years ago depending on the franchise.
Increased modal share by car is fine if there's a fall in traffic to accommodate it (which there had been to date, most noticeably due to working from home if you are able advice and schools not being fully back) however it wouldn't take a lot for the roads to start to get busy again. Once they do that's likely to discourage driving once again.
Therefore whilst cuts have been needed up until now (which have happened with reduced timetables) I suspect that (subject to no significant second wave and associated lockdown) that from next year whilst there may be a need to not being into service the 442's (as an example) overall there'll be little which might actually be needed to happen.
For instance things like rural branch line closures would be very politically hard to swallow and probably wouldn't see that much of a fall anyway.
Likewise things like the new trains for Avanti are likely to be delivered after rail use is back to where it was when they were announced (maybe 12 months before), however certainly no where near the levels seen at the failed franchise bid for ICWC about a decade prior to their delivery.
Actually the fall of Flybe could result in positive growth in some rail services. Especially if it results in an overall increase in cost due to there being less competition. Even if that extra cost is just down needing to get to/from a different airport than was the case before.