The cast were told a few weeks ago that their contracts were being extended so I don't think it's a spontaneous marketing tool.
It's a pretty cheap show to run by West End standards (actor-muso, so no additional musicians' fees, simple set, etc.- and Bill Kenwright is not noted for paying his actors too much LOL). And I think it's selling pretty well on weekends - it was full on the Saturday night I saw it.
I actually only went to see it coz a friend of mine is in the cast. But I have to admit that I really enjoyed it! (And I have an aversion to jukebox shows.) It is genuinely funny, the songs are all great and every member of the cast is brilliant. The biggest revelation was leading man, Scott Bruton (of X Factor fame). He had immense stage presence and natural charm and absolutely nailed all his vocals. It's a real crowd pleaser of a show and I'm not surprised this little guilty pleasure of mine has been extended.
THEATRE 2020: CURTAINS**** LET'S HEAR IT FOR THE GIRLS***** WICKED***** KEITH RAMSAY TAKING NOTES WITH EDWARD SECKERSON***** KAYLEIGH MCKNIGHT CONCERT***** RAGS***** ON MCQUILLAN'S HILL** DEAR EVAN HANSEN***** THE JURY***