That's a grim thought - they didn't have to resort to fillers like this last year, The Sky at Night plus various Proms, Sports etc covered for the Yewtreed editions- so either there's less sport around in 2017 or we are going to see less shows than ever
What I think might have happened is that BBC4 didn't schedule TOTP for Friday, thinking that they were going to be showing the Eurovision You Decide programme as they did last year. That Eurovision programme has been moved to BBC2 for this year, presumably because the BBC want a higher profile for it. So rather than having to disrupt the scheduling of all the other TOTP's, they have just slotted in a Sounds Of The 70's in it's place.
True, but the schedules are worked out a long time in advance and I think they felt it easier to slot in a Sounds Of The 70's rather than shuffle all of the TOTP episodes forward. It's just a theory though.