Fun fact, it's been done 20 years before when Surinam Airways received a B747-300 from KLM which became PZ-TCM. The aircraft was brought back out of storage & was given a heavy check by KLM in AMS before going to SLM. Here too a lot of techinical delays were eventualy suffered en the same talk of retiring the aircraft was around. This was with KLM support in AMS. When it finally was retired after about 3 or 4 years KLM had to do another check before it could be flown back into storage. According to a friend of mine who then worked on the aircraft both times, it left AMS in a good as new condition but came back in a state that was very much different.
With much the same on the A340-300s that followed now it's this B777's turn. I assume you get back what you put into it and if that's the case then the next generation is eventually not going to do much better. Let them get their B787 and see how they manage with that over the next few years.If it runs down the same then it's probably better for everyone to continue a wet lease with a 3rd carrier instead of operating own metal.
The SLM 747-300 was operated after all 747-300's with KLM were already retired. So the pool of licenced ground engineers became smaller, also the spare part position deteriorated.
IMO , the only way to operate a longhaul fleet with only one aircraft is to have the same aircaft type available as back up at one of the two destinations (PBM or AMS).
Because PBM has practically no other long range traffic only AMS can be used for technical support and as back up aircraft location.
Also the same type of aircraft must be fully operational with the supporting airline (KLM, TUI, etc)
In the past the perfect cooperation SLM-KLM was during 1975-1983, one aircraft (PH-DEM) of the KLM DC8-63 fleet was leased to SLM in full SLM livery and if not used on certain days was operasted by KLM. If the SLM aircraft was down for maintenance , another DC8-63 was substituted.
Operating a twin over the ocean, you're always one engine failure from a total emergency.