There are any number of big naked bikes out there...
There are some large scooters which sell better than anyone on here would have predicted a few year ago.
What keeps many car users off two wheels is that they see no reason to spend similar or more money to sit in the rain for a marginal improvement in fuel economy (if any, see the VW polo diesels for instance), potentially more expensive road tax and higher general running costs.
It's a no brainer, Polo diesel= practical warm sensible cheap, bike= cold wet expensive, and therefore viewed in the same way as a jetski.
Until that changes then bikes will never become mass market, they are developed and sold as toys, in the bad/good old days bikes were cheap, slow and did 100mpg, cars were expensive, slow and did 30mpg, hence sidecars and family transport being the bike.
Now cars are cheaper in relative terms than ever before and bikes are (mainly) stupidly expensive to run. If you keep your XTZ for 2 to 3 years then kick it over it will probably be a write off, crazy

The current crop of bikes don't cut it as mass market transport, even if fuel does make it to £10 a gallon they still wont, but something can/will... it could be two wheels but as long as we copy a fairing spec designed by the racing authorities in the fifties to be inefficient it wont be bikes as we know it.
All imho of course
