Up thread someone mentioned the cost of new or refurbished vehicles and someone else mentioned 40 to 50 days use per year. Those two things really encapsulate the problem.
£ in very large numbers to solve the problem.
I think we have missed the chance with mark 3's as those which are left available probably need large sums to sort out the corrosion under the toilets and step areas. Just look at the delays to XC, Scotrail and GWR refurbishments and power door installation. Opening windows would not actually be a problem - the windows are very close to 455 and 150's and hopper vents are available.
Saphos have an owner with very deep pockets, which is how they have restored so much stock to superb condition. Likewise WCRC have external funding from their owner. Upmarket operators like Northern Belle and British Belmond Pullman have a multinational backer. It's the the other players who are looking at problems with CET and over-ride strengthening being the final game changer.
At the moment outside Saphos and West Coast and "posh dining operations" there are not many sets of stock suitable for charters, Riviera Trains, Vintage Trains and SRPS. That will limit the railtour market almost more than loco provision.
The business case for loco hauled coaching stock is appalling, maybe 40 weekends per year plus ECS movements to and from their base. Probably 300+ days per year collecting seagull c*** on the roof and not earning money.
The 5 WES idea was kicked around within the industry in the late 1990's as an idea for short and cheap steam shuttles. No need for a T&T loco as the WES could haul the the steam loco for a short leg like Southampton - Eastleigh or Portsmouth - Havant. It even made good use of units not needed on Sunday mornings before Sunday evening formation strengthening. The trail went cold when SWT found they couldn't operate steam traction under their franchise agreement and would need third party involvement. Then it took longer to restore the suggested loco due to other problems.
In conclusion, stop donating to restoring locos for a year and rebuild mark 1 coaches for a couple of years. Just expect small players may drop out of the market in 2023 when the CET and door locking regulations apply. Whatever happens the market will be increasingly dominated by those with very deep pockets......