I agree with PDR1
In my experience trying to manage a large new, IT heavy, aviation project, it is obsolescence of COTS software that will come back to bite you. Literally hundreds of separate but interdependent software products may be required.
If you own the overall product then you need an in-house organisation to track obsolescence and manage the impacts this does not come cheap.
IMHO best buy the entire product, with the software maintenance task,and let industry do the dirty work of keeping it all together and functioning.