That's your conjecture (albeit not one without legs), not a proven fact. I know people who prefer LUMO's seats (I do too).
Yes, but why would you use a £200 Anytime ticket on Lumo when Lumo's fare is much lower? OK, more than would now you can't refund on the day (when you could there was really no reason to ever do that, you would just refund it and purchase the Lumo ticket) but it's only going to be a tiny number of people - basically largely business travellers whose company is willing to just buy them a couple of swingeingly priced Anytimes rather than insisting on Advances as most businesses now do plus the odd very well-off leisure traveller who is willing to pay a significant sum for total flexibility.
I do agree that Lumo's seats are better, by the way, but the legroom is worse, the luggage policy incredibly tight (very rarely will I be making a long distance journey without more luggage than Lumo permits) and the whole ambiance a bit tacky and "down with the kids", whereas LNER feels more professional and has better policies. But I do get that different people prefer different things - I'm just not convinced that GBR should need to make concessions to OAOs in ticketing terms in the future model, because in other countries the state operator doesn't.
Having just experienced some full-and-standing intercity trains in Poland, I couldn't help but reflect on my (purely anecdotal) experience that in the UK we tend not to do revenue checks on full-and-standing or late trains. This would potentially put LNER at a disadvantage. Other posters have given reasons why stats derived directly from ticket scans are likely to be volatile and unrealistic. A system needs transparent and stable parameters - that's not to say ticket scans can't be part of the calibration process.
When a state participates in a multi-player market, it should not set rules that make things less onerous for itself vis-à-vis other participants. Such would be despotic behaviour.
Well, there's always the option that Lumo has to do everything itself with no acceptance of GBR tickets nor sale from GBR's site. That's how it is on the European mainland. Giving them a concession of being able to accept and be paid for GBR tickets on the condition of scanning them to prove they did seems entirely fair and far more than GBR has to do.