My local Kubota dealer had no problems finding parts for my F2880, but it's not really all that old IMHO. The rear end had a couple of bearings that were bad. I tore it down as far as I dared, but there was one more bearing that needed replaced and it was 2 more assemblies deep, and it looked like putting it back together was going to involve setting the gear mesh which is not something I have enough experience to trust myself with using the tools I have. I took the sub assembly (and some the other parts) to them, and they replaced that bearing and reassembled most of it for me. The other side was in better shape, so I was able to install those myself.Anyway, I don't know if JD zero-turns have problems like this, but Kubota people say Kubota is great about supporting old machines, so maybe it's a good idea to look at their stuff. I have never had a problem finding Kubota parts for a 2003 machine, and some of the parts are cheaper than John Deere by an insane margin.
Also, my Kubota has never broken down or given me one serious problem. The steering cylinder needed repacking, but that's normal. The JD is sick all...the...time. Alternator. Injector pump leak. Hydraulic cylinder failure. Belt broke, which is a near-disaster with this design. PTO switch failed. Broken muffler.
You can't tell a customer a product is a lifetime purchase and then quit supporting it. This tractor was a good 20-year-machine, but that's about it, even for a homeowner. If you want to go longer, you will have to find creative solutions and spend money.
I'm thinking of getting a gas Scag. I like diesel, but I don't like spending $20,000 for a lawnmower. By the time a good gas mower has serious problems, I'll be past mowing age.
I think the older Kubota stuff was pretty good. Some friends of mine bought a new BX25D around 2012 and had nothing but problems with it. With today's emissions requirements, I wouldn't trust anything with a diesel engine these days. I used to hear REALLY good things about scag, but I haven't heard or seen much about them for several years now. Unfortunately, I'm not quite old enough to make it with another gas machine, especially since I tend to buy used machines. If I can find a pre-emissions diesel with reasonable hours on it, it would likely last me long enough though.