At the root of the problem is the dire product support and level of investment in the technology. Great aircraft but just how long can the customer carry it? And they seem to be carrying it on a day to day basis.
Its been ten years now and its been a lurch from one thing to another. If its not the NOTAR its the rotor head etc etc..
The big mistake along the way was probably that decision not to let Bell buy into MD. That decision has undoubtedly hurt both parties. Would the 429 have seen the light of day were Bell to have had the 902 - with or without the NOTAR - to play with and develop? I doubt it.
Those 135 tail rotor problems [and I am not sure whether they actually led to a general grounding as such] were way back ten years when the first 135 was introduced to UK police service. The low numbers in service then may have masked a grounding.
I accept that we are currently facing the demise of the 135 Classic in police service because it has no autopilot... the difference though is that this is planned two years away and was not a requirement when the aircraft was bought.... That said - perhaps thanks to good development support - the 135 has had a charmed history. I can recall BO105 groundings but never yet a 135.
I do not buy helicopters, but the people who do buy vote with their wallets and we both know that that vote says something like 100 Explorers, 600 EC135s and a similar number of that ancient mariner of the light twin market the 109 [depending on whether you include the 109A].