See my other post about heathrow looking for more shuttle flights to and from Glasgow/Edinburgh.
If the rail journey gets below three hours station to station by full High Speed or classic compatable trains using HS sections, demand will change.
Airport slots could be used to connect with other cities instead of being spokes to a greater hub.
On Eurostar, it’s a pity the original idea of ‘regional’ services never happened and sad to see that the trains intended for these services were recently reported as surplus to SNCF’s needs.
But, maybe one day continental destinations might be possible from the grand old terminals of Glasgow and Edinburgh.