I don't think that anyone ever claimed that changes in passenger numbers were uniform - it'd be weird if they were uniform - of course there are variances (just like, when passengers numbers were rising each year, they weren't rising by exactly the same percentage nationwide)
I'd also point out that... it's December 2021 now... a number of people were assuming that numbers would bounce back after the first lockdown, or the second lockdown, or maybe after the 2021 summer holidays ended... but things haven't bounced back yet... I keep hearing that people working from home "would like to be back in the office at least two days a week" and all that but... it's getting close to two years since the first restrictions happened ... so if not now then when? If you're excusing the current figures as a "blip" then when will people be back on trains and in offices? January 2022? Next summer? I could buy into the argument that things would be back to "normal" come September 2021, once the restrictions were ended and summer was over
It matters a great deal
1. Commuters are predictable. You'll have slightly fewer on a Friday on some lines and slightly fewer in August, but generally it'll be fairly stable. Whilst there's variation, if the trains have sufficient seats for a typical winter Wednesday then they'll have enough for pretty much every service of the year
2. Leisure traffic is not predictable. How many people will want to go to Edale/ Whitby/ Windermere will vary significantly week to week, be heavily weather dependent, be hard to predict when planning stock availability/staffing rosters? And the stock (and platform lengths etc) required to meet the peaks demand on a handful of warm/dry Bank Holidays will be over the top for over 90% of the year. You might get some large groups (sometimes a one way journey as part of a walking trip) who'll overload your train
3. As pointed out by others, the costs of providing a leisure journey is significantly higher than a typical commuter. For example, a commuter may buy their season ticket once a year and never use a booking office for twelve months. A twelve coach EMU can carry several hundred commuters with only a driver and possibly a second member of staff (but not necessarily one on a "safety critical" wage). Spread those hundreds of passengers over a dozen different branch lines and you've got much higher costs (leisure passengers are more likely to use booking offices for each journey, branch lines obviously have a much higher "cost per seat" than a commuter EMU
4. I'd wager that an amount of the leisure passengers we've had over the past eighteen months have been because of international travel restrictions - i.e. people heading to Blackpool who'd normally have headed to Benidorm - working from home will be "the normal" for a lot of people a lot of the time in future years but I'm not convinced that the demand for trips to the British seaside will be so bouyant once people are able to fly to the Mediterranean instead (I knew a few people who've taken trips to "new" UK destinations in the past year or so who'd normally have flown far away for their holidays - no guarantee that this will be the case if there are no restrictions on international travel)
5. Like it or not (and I don't particularly like it), commuters into London are very "visible" to Government - MPs and senior Civil Servants use those trains themselves - it's hard to suggest cuts to those services. People heading to the seaside are a lot less "visible" in that regard - so it's a lot harder to demand generous settlements from the Treasury, especially when the railway is seen as less economically necessary to the UK economy
...because we see a need to provide kids with education or to provide people with a certain level of healthcare
Whilst we want to provide public transport, if insufficient people are using heavy rail then it's no longer cost effective (so we look at either reducing it or providing public transport by other means)
Providing heavy rail is a means to an end, rather than a goal in itself
Society requires every to be educated to a decent level and taken care of when ill -society doesn't require every village in the UK to have a branchline
There's a huge difference between "we should focus resources on where best appropriate" and "we must pare the railway network back to only a profitable core" - I don't think that anyone is arguing for the railway to be profitable - it's just that different people have different thresholds of where heavy rail is appropriate or a sledgehammer to crack a nut