Guards not doing tickets isn't solely down to doing doors though, neither is it just because they are too lazy to do them. Many TOCs number one policy around doing tickets is to 'avoid conflict'... How the heck do you check tickets, challenge people with non-valid tickets, whether deliberate or not, or get money out of druggies, drunks and yobbos, without getting into conflict...?! If you use the powers you're given under the byelaws to enforce anything the BTP will probably commend you while your TOC tries to sack you! Even if this wasn't the case, how can lone Guards be expected to deal with these people while RPIs normally patrol in at least pairs for safety. Now, I'm looking at this from an urban/suburban point of view, but you get these situations on all trains. I can't see this is going change by converting Guards to lone TMs/Ticket Inspectors, other than these TMs being more conveniently dispensable (operationally) than Guards should things go wrong.
Another thing I'd say is that people on this forum and many of the fare paying public care far, far more about revenue protection than most TOCs do. Revenue collection/protection is considered an expensive luxury by many TOCs, particularly where it involves having to pay someone's wages to do it. Some of us have seen Clerical, Booking Office and Revenue Protection grades almost wiped out in our TOC when they have wanted to cut costs, as such many of us Guards know that we're next to be disposed of once we're no longer operationally required and have no power to influence any decision making. Some members here might think that we'll justify our post Guard existence by selling Tickets and Kit Kats, but most TOCs most assuredly won't and I doubt the DfT will in the longer term. Right or wrong, the only person you need on a DOO train is the Driver, anything else is a luxury.