Used to spend £5k man isn't spending that any more - revenue is down.
Passenger numbers are down 30%. But if you can get them back up, it doesn't matter if they are leisure travellers or commuters, as both pay about the same. Commuters are a solid income but they aren't a goldmine.
What are a goldmine are occasional business travellers as they do tend to pay higher fares. Reduced commuting is likely, in time, to result in increased numbers of those as people go into the office maybe 2 days a week.
Let's use Bletchley-Euston as an example:
12 month season is £4904.00. There are apparently 255 working days in 2021, so if we assume 25 days holiday which is pretty average that means 230 working days for your commuter. That's £21.32 per day.
The Off Peak Day Return is, wait for it, £21.30 - exactly the same, pretty much! There is a Super Off Peak at £16 but it's not valid until the afternoon including Saturday restrictions so is only really much use for evenings out or Sundays, so is a minority "headline fare" ticket rather than commonly used. (There is a way to pay that fare without the Saturday restrictions but you have to know what it is and so most people won't do that, so it's even more niche than actually using the super off peak).
The Anytime Day Return, however, is £42.10, which is very close to twice that. So if that previous season ticket holder goes into the office twice a week but paying that fare (yes, there's a way to get it cheaper, even more so if you have a Network Railcard, but again most people won't) is bringing in £84.20 as against £106.60.
This is a reduction, but not as much as you might think, you only have to sell one and a bit more leisure travellers an Off Peak Day Return to pick it back up (and you'll sell the odd few more to now non-season holders who might buy the odd one on a weekend instead of using the season they used to have). And if you consider that there is likely to be a move away from London due to not needing to be in every day, the overall take might actually end up higher.
There are various complicating factors e.g. monthly and weekly seasons and Railcards, but this should give an idea.
(I deliberately didn't use MKC as this has lower leisure fares, but that's as a result of direct on-rail competition, which is available at a tiny minority of stations - the majority of South East commuter stations will look more like Bletchley)