As said in a previous post, there's policy, and there's reality. The Huawais of the world can amortise the cost of a 'software support facility' for want of a better term over millions of sales. It's a small sum under 'miscellaneous' in the investment appraisal. If you have a fleet of a dozen aircraft, not enough crews, maintainers, and all the other resources you need, a honking great % of your budget on an SSF, where you can't guarantee retention, or expertise in the first place, is madness. The rules are that you must continue to have the Design Authority under contract anyway, so you're paying double. There are exceptions of course, but as S-D says the subject needs careful thought and assessment.
The last time I managed a programme where the Service wanted a SSF, the decision was easy as they hadn't bothered costing it or including it in the endorsed requirement. On another occasion I inherited one where 3rd Line had set up a SSF. The first time a Fault Investigation was sought, the company fell about laughing. You've been faffing around with the design, the fault is in a build standard we dont recognise, and aren't contracted to support.