Wasn't ITSO enabled on London Underground barriers relatively recently? Not nessarally a full replacement but certainly one that works for some usecases.
It works fine on the Glasgow Subway, which also prints paper ITSO tickets at a higher expense than using a smartcard (although not as extreme as the Tube paper single/return fares), but with TfL…
…I actually don’t have a good reason for why this wouldn’t work. It also gives an option to retain paper LU single/return fares (at the extortionate rate) whilst still retiring the magstripe readers at the gates.
On another note, how are you meant to get in a Compulsory Ticket Area (CTA) in a station where the platform tickets are sold by LU only (such as West Ruislip) if the platform tickets are withdrawn? Do you have to have a Travelcard or tap in and out and pay for a same station exit, which would be either the minimum PAYG fare for 2-29 minutes or two maximum fares for 30+ minutes?
Of course, the National Rail approach at non-CTA stations (aka the vast majority of them) where you just ask the gateline staff nicely doesn’t work when you get charged a Penalty Fare for not having a valid travel ticket or a non-existent platform ticket.