Upgrades may be close to impossible (unless the owner has its own technology), but the spare parts will not skyrcocket, if the buyer has any sense and negotiation power. They can store the critical parts and manufacture pirate parts. In military sector the inspectors do not check the originality of the parts. At the end you may have to cannibalize some planes for spare parts. That happens even for civil airliners, even quite new ones, if the manufacturer sells planes with dump prices to later recover the profit from spare parts.
1. You obviously have not heard of DMSMS.
2. "inspectors do not check the originality of the parts"? Seriously? Parts are ALWAYS checked to ensure they are original parts, along with the attending paperwork. And the military is even more strict on the originality of parts.
Any hard facts? There are not that many users: Australia, Canada, Spain, Finland, Malaysia, Kuwait. Who has problems?
The USN and USMC for starters. Did you notice how many classic Hornets are out of service awaiting spare parts with the USN and USMC, and how they are often robbing serviceable aircraft from stateside squadrons to make sure squadrons overseas and on deployment have serviceable aircraft? I think the phrase 'death spiral' is being used in regards to serviceability of the Hornets.
Also, by 2020's, the mission computer on the F/A-18 Hornet (classic Hornets) will become obsolete. The USN, Australia, and Canada will not be paying for any more upgrades to the mission computer on the Hornet, which will lead to the only user operating the Hornet past that time frame, Finland, to shoulder all of the development costs associated with keeping the mission computers updated. And there are a total of 46 equipment-specific software-driven systems, with 8 of them require software updates throughout the service life of the aircraft. And the costs of software updates have continuously increased as users start phasing out the Hornet, and, hence, no longer participate in sharing the costs.
Or, if you want something structural, how about the centre barrel on a F/A-18? Replacing that on a Hornet will set you back roughly $3-5 million per centre barrel... and that's volume pricing!
Even for fighters still in production like F-16 the oldest and newest ones are not 100 % compatible with spare parts and the early version may suffer equal lack of spare parts.
The majority of F-16's out there are of the later Block 40/42 and 50/52 standards. The USAF has retired the older blocks, and has updated the Block 40/42 aircraft to a similar configuration to the 50/52's.
Also, the majority of the earlier F-16's out there have undergone extensive avionics updates under the Mid Life Update program... the avionics on most early model F-16's are equivalent to a current production F-16 Block 50/52.
Which spare parts you have to buy in bundles and cannot store or produce locally or buy from alternative sources?
Any other country than Switzerland and Finland even plans to use Hornets for the next decade?
Everything. It is not cost effective to ask GE for example, to produce a handful of F404 engines for one user, if GE is even willing to do so in the first place. The cost per unit would be astronomical.
They might be willing to keep a production line going if they have an order for hundreds of engines on the books, but they would probably shutter the production line if they don't have enough critical mass to keep production going.
Also, tons of parts on the F/A-18 are ITAR controlled, which means you will need US State Department approval to even touch them... the parts can only come from the US government approved sources, and you can't reverse engineer or copy the part, as that is a breach of ITAR compliance regulations... which means a ton of legal consequences for everyone involved.
And the Swiss intend on retiring the Hornet after 2020, and Finland will also be formally selecting a new fighter in 2021.