Merc is doing fine with CLA, C coupe, E coupe, S Coupe, CLS, SLK, SL
BMW currently has 2,4, 4GC, 6, 6GC, 8 and upcoming 8GC
Audi has the TT A5, A5 sportback and A7. I think they could introduce a new coupeish model without A) axing the A5, and B) ruining the TT name.
Where is the 2- door A7? That would eat up sales of the E coupe and 6. Where is the followup to the prologue. A true rival to the 8 and S coupe? VAG has nothing in this segment. Porsches are a different league, bentley goes against RRs, maybach's. Audi is the brand to compete with BM and Merc, so no point saying the bentley Conti could go against the big coupes.
They have all the tech,all the platforms, all underpinnings, the interiors. They'd just need to design bodies for these cars. There is a market, and they just give up market share. Just like they do with RS models in the US.