It isn't that predictable. There is quite a flux of demand, even weekday to weekday.
It's very predictable. It's not always the same on the same time at different days, but if you look at the pricing on the easyJet website it isn't now a case of it going up over time quite as much, they tend to predict what they can charge for a given flight months beforehand. Premier Inns are similar, I just booked one for this weekend at a fairly reasonable rate which is no doubt very close to what it went on sale as (and the website said "last few rooms" at that one, I don't know what triggers that though), and similarly if it's a school holiday they don't go up at £29 for the first few rooms, they start off at £150 and stay roughly around there.
Some things can affect it like school holidays and major events, but again those are inherently predictable.
What I do agree with in the OP's principle is that the £10 admin fee for Advance changes needs to go*, and ideally if you switch to a cheaper train (thus doing the railway a favour, because it'll probably easily resell the more expensive seat) you should be able to get an e-voucher for the difference. It used to be the case that you'd just buy a walk up ticket if you wanted flexibility to change plans, but this is no longer within reach and so given that the purpose is more granular pricing rather than getting money in advance it doesn't make sense to clout people for changing plans. Premier Inn now doesn't charge a change fee and refunds the difference (as actual money) if you change to a cheaper option.
In short, if the future is Advances for everything, Advances need to become more user-friendly and not still be seen as bargain basement, super-restricted tickets.
* Maybe allow refunds and keep it for those, or allow a percentage refund, say 80%.