Maybe the sale of Arriva by DB in Germany slowed down investment.
I thought the investment stopped not long after DB took over...
The investmeant stopped when DB gave arriva stupid targets ho meet that DB knew they couldn’t reach
Sorry but there's a few things that need a bit of clarity in the interests of fairness.
Arriva was purchased by Deutsche Bahn in 2010. Now Arriva already had an issue in how it accounted for individual route performance so those "stupid targets" were already an issue which predates DB. You just have to see how many depots were closed in the period 2000-2010, and the paring back of the network e.g. the large scale abandonment/sale of much of the former London and Country operation.
Despite this, there was fairly consistent and healthy fleet investment during the early/mid 2010s as the impact of DDA/PSVAR was being felt. Hence why main fleets have a substantial number of Pulsars/Geminis and e400s of that age. However, it became apparent that DB had fundamental issues. It had purchased Arriva and Schenker (logistics) as it sought to diversify which had added to a debt pile. It needed capital investment in its core business (which has well documented issues) and so investment elsewhere was limited. One off-quoted example was that a UK bus subsidiary was competing for capital for fleet investment against a bike hire scheme in Bratislava - that got the cash as it was a contract backed scheme (guaranteed income) vs the commercial risk in the UK!
On top of that, you had the protracted will they/won't they sell or spin off discussions. That meant managerial paralysis and continued retrenchment/inability to invest. Covid then simply exacerbated a bad situation.
It seems that under new ownership, there has been a bit more capital investment but there are some fairly fundamental issues with the business. Some operations have withered away and are a shadow of what they were e.g. Northumbria, the former Midlands North, Shires, Southern Counties. How or if they can be rebuilt is questionable.