Morning Thomas
Welcome to the Forum
I went through the same dilemma a long time ago, my X5 4.6is was reaching the end of it's extended Warranty what to do?? Well after trying new X5's (E70's at the time) and a few other Marques F Pace Jag, ML Etc, I decided to keep the 4.6. As an Engineer I have always carried out a lot of work on my cars myself driven by a need to know and a mistrust of some dealers (that's another story). I found a local BMW Specialist who I use for annual service/MOT and of course keeps the service history up to date. I also took out an after market Warranty in case something major went wrong 1/3rd of the cost of the BMW cover and when needed were excellent. My X5 is now 18 years old and still going well I inspect the car once a month under and over plus a diagnostic check to make sure it's in good health. If anything shows up I make a decision on whether I will fix it or get the garage involved.
My original decision to keep the car was based as an Old retired bloke on the cost of replacement against the cost of running on a car I really liked. Long term did I make a good decision ? I think I did for two reasons if I had stuck with my (up to the 4.6) 5 year upgrade I would have born the cost of 2 upgrades by now were as all I have had is fuel and running costs. Another plus is the fact that the X5 4.6is was rare in 2003 only around 800 registered total in the UK the last time I checked it was below 300 so maybe one day I will have a classic with 156k on the clock and it still leaves hot hatches dead if needed.
Only you can make the final choice but if you really like the car I would consider after market Warranty to protect your wallet and run it on.
Dave