I have left bike bags at airports (Venice-Copenhagen-Stockholm-Athens x 2 - Olbia, Sardinia) in the luggage facilities – It is not cheap – but something I had budgeted for and I could guarantee to some extent that my bike would go on the plane on the return trip).
When the Eurobus had a better range of destinations – Venice – Frejus etc it was a very attractive way of travelling - no hassel, simply get to the pick up point and climb aboard – it has always been expensive and on the face of it time intensive – but I would still recommend it as an option for French tours – trouble is that flying for a decade or so looked to be the most economic option – and with the number of destinations much more exciting – I think people migrated away from the bike bus and instead of it expanding its services it has become a shadow of its former self.
In recent years now that it is easy to look up things on the internet I have been exploring emailing campsites near my chosen airport to see if they will store the bags for a small fee.
You're probably right about the bike bus and it's relative demise - what a shame. It's also a pity that it isn't made easier to use European cross country trains.
I like the camp site left luggage option but on the last couple of tours I have done an A to B rather than a loop. There is always a solution however with some ingenuity.
I wonder if I could buy a trailer to hitch on the van and start a small scale Bike Bus enterprise!