The issue is mainly the companies which provide spare parts in the first place; manufacturers and aftermarket parts suppliers struggling to get spares imported and distributed in a timely manner. Warehouses that should be full of spare parts for common types of bus are empty. And with a limited supply, the price of what few parts are available goes up.
If anything the mass popularity of models like the Enviro 200 MMC make the problem worse, as Stagecoach, First and a hundred other companies are probably competing over a limited number of spare parts for the same type of vehicle. Crash damaged or broken down MMCs are mostly still too new (from a financial/investment point of view) to strip for spare parts to keep other vehicles going, so they sit around waiting to be repaired.
Meanwhile, there's a reverse logic for operators with an older fleet. While operators with the latest ADLs and similar are sat waiting for overdue orders of spares to come from China or wherever, parts for older buses like early 00s Solos, L94UBs, early Omnidekkas etc can be sourced from the Barnsley breakers. Anything of 'withdrawal age' would be far quicker to get parts for!
The first paragraph suggests that manufacturers have a key problem. If companies buy new buses from them, with inadequate spares support, will they continue purchasing them. You would have thought those manufacturers would step up, if they wanted future business. With limited supply, surely a company holding key stock in a timely manner, means they are held to ransom when there are shortages. Lead time, safety stock, capacity considerations, and suppliers adhering to terms of agreed contracts....unless of course it is all placed "spot" as and when the need arises.. The second paragraph possibly indicates why so many, secondary operators are opting for the Volvo/MCV. But is spares provision any better. The third paragraph certainly highlights why 16 year old buses are going through overhaul.. Easier to get parts, even reconditioned ones. Maybe even Volvo and Scania are more robust. Overall, it raises the question "Is ADL is right supplier?". Maybe recent order commitments and indeed future ones in 2025 will show. You can imagine a conversation with ADL "Well if you had had a reliable supply of parts , we would have given you more orders. It will be interesting to see how Yutong deliver. Given how many vehicles they supply worldwide, every year, they must have a ready supply of parts
Perhaps someone can explain why Wellglade is/has been so badly affected when most other companies recovered a while back. It's not as if Wellglade has a lot of 'odd' or unusual vehicles, in fact they're more standardised than many others. Strange.
Usually the case that the companies that no longer have a problem, have put a meaningful and deliverable action plan into place. Maybe their margins are better. Maybe they have more preventative maintenance in place. All, I can say, is I've never seen anything like it, and I worked in supply chain management, in a strategic role for over 40 years. I could see a situation where the money isn't there to put it right. Either way, when you have an unreliable delivery in a business (no matter what sector), customers drift away, which , over time, makes resolution of the problems a lot more difficult.