I agree we don't need them when the operators start accepting contactless (has that happened yet?) - the vast majority of passengers will likely either be on some kind of season ticket, or use contactless, both of which are fast.
However, there was no sign of the bus accepting contactless when the already existing ticket machines were disabled back in 2018 or 2019, and people buying tickets with cash from the driver is a very good way to extend dwell times, and therefore make the bus less attractive.
My point was that at the time the off-board ticket system was removed, without an equivalent introduction of a fast method of paying when boarding (like oyster or contactless in London), seemed like a pretty insane decision, especially as the infrastructure was already in place.
There was a period (maybe in 2015 or 2016?) when both whippet and stagecoach ran services along most of the busway, and at least some interoperable tickets were available.