Yes, the through tickets to the airport still work, I used it a couple of weeks ago. Marketing/publicity is terrible. As you observe, the Scotrail site says Arriva bus but it's actually McGills. The timetable outside the station at Paisley Gilmore St lists the stops including "Caledonia Way". You need really local knowledge to know that's the road past the front door of the airport - there is no other indication that the bus serves the airport.
A cynic might suspect collusion on poor marketing to make casual visitors think that the (more expensive) airport bus from Buchanan St is the only option.
Back to original question. I needed to travel from London to Glasgow last week, booking with only a few days notice. Flights with BA were cheaper than rail (the advance fares were almost same as standard off peak) - which is not what I'd expected to find with a short notice booking.