Ferry is a minimum of 2 hours. Factor in checkin-in a minimum of 30 mins before departure. 90 mins drive Cairnryan to Glasgow centre is ambitious, doable provided you are first off the ferry and don't get stuck behind anyone on the A77. Also taking your own car massively ramps up the price and the minimum return price for that is £148 usually, unless you purchase it in the sale.
BFS-GLA - arrive at STD-40. 45 min block time. Bus to Glasgow City centre - 30 mins. And unless you're booking last minute, still cheaper than £150.
As for demand... I assume you've seen the amount of freight that uses the ferries? Plenty of times I've used P&O and Stena Line and there's only ever been a handful of passengers, the rest are commercial drivers.