This may be a dumb question but I often wonder why sensor sizes still equate to film sizes
Actually they don't, eg APS-C, M4/3 plus all the smaller sensored compacts.
Where they do, it means you don't need to design new lens designs and can re-use existing designs.
whether or not we might see sensors grow to 6 by 6 or even 6 by 7. On medium format they seem to be stuck on 6 by 4.5.
Rather unlikely. As the size of ICs increase manufacturing costs escalate hugely, then add that few people actually want bigger cameras, bigger lenses, so as volumes fall costs again increase.
35mm Full frame appears to be a sweet spot, as sensors are affordable, cameras are of manageable size and quality is high enough for the vast majority of users.