Aye Kreece,
All depends where you live in the world and if you're viewing motorcycles as an "investment"?
Even Broughs, Vincents and Velocettes are levelling off in price or reducing. If it is a Japanese bike, with an electric start and you are in the 50 to 60 year old age range, then money still seems to be no object for specific models.
However, if the bike requires engineering skill to maintain, a kickstart, or run and bump to start, multiple levers to juggle, then only the die hard enthusiast, rather than the "bean counter", appears to be still in the market. Hourly rates for skilled labour are increasing, as they become more scarce and many Japanese machines over 30 years old, have few OEM spares readily available, as they have been discontinued. Also, the EU requirement to not hold stocks of spares for any machine more than 10 years old, contributes to the inbuilt obsolescence process.
Personally, my machines are to be used, worn out and maybe rebuilt. My 450 MKIII that is part of the family, has been on my life's journey since 1976. My wife's Bros 400 has likewise been with us a long time. I also have SRX600's and a Moto Morini all have been refurbished in part or whole, and had money spent to keep them in running order. There comes a price when I will cry "'nough!", I haven't reached there yet for my motely acquisitions, but haven't bought anything for over 10 years now and if I did it will have to be for less than £2,000 for a good runner. Also, when I pass, my machines are to be sold, not hung on to and corrode in storage.
Restoring a motorcycle to hang on your living room wall and corrode internally has always struck me as "oddball", as does paying over £100,000 for a decrepit Brough that is more iron oxide and fresh air than motorcycle!
Wear them out and encourage the iPhone generation to give the phone a Whee! into a loch and swing their leg over something analogue rather than digital, although I would make an exception for ignition systems!
Good health, Bill