Most people would rather have 100% of something than 85% of it.
OK fine, you build a website that achieves that
Trainline charge a flat fee and that can work out a lot cheaper than 15%.
Trainline charge a booking fee for all transactions, except where the app is used on the day of travel (and maybe they've extended it slightly; I am sure you will be quick to identify any other circumstances where a fee is not charged).
Trainline do not charge a share of savings, but if there is no saving, then you are unnecessarily paying a booking fee; most short distance journeys would therefore attract fees unnecessarily.
Furthermore, Trainline do not typically find cheaper itineraries; they tend to only identify the fastest itineraries (and direct trains) and then price them up, rather than go looking for cheaper options.
And in addition to all that, the last time I checked Trainline only do one split; they may have changed it again but even on occasions where multiple splits were offered, it was rarely the cheapest in my experience.
As you say, these things cost money to work out, which is why Trainline & Trainsplit both charge fees.
You are very selective in your wording but I don't think it's right to say "charge fees" without context; anyone can use Trainsplit without being charged any fees whatsoever if they turn splits off (i.e. it will offer the same fares as the TOCs, plus cheaper fares for slower itineraries). The only "fees" are a proportion of the savings made by not using a TOC site.
Sometimes the fees of one is cheaper than the fees of the other. Sometimes the fares of one are dearer than the fares of the other. It pays to shop around, the best money saving is made by shopping around, not just blindly using one site everytime.
Yes some people have the time to use multiple sites to shop around. However I would point out that when making any journey, or series of related journeys, there can be very big advantages to using one retailer, in case of disruption to any part of the journey. I'd also aim to do it in one transaction.
If you use multiple sites, you should also ensure you have a record of exactly which site was used for each journey, in case of any issues.
Also you may wish to consider after sales support, especially in these uncertain times with strikes, overtime bans and last minute timetable alterations.
But not everyone is price conscious, and some people will be happy to only get 85% of something
Then turn splits off and avoid any 'fees' whatsoever, while still benefitting from the cheaper itineraries which are slower. Simple!
It's just one click: