According to you, most of the air forces which keep their aircraft flying must be doing it by magic, since they can't possibly be able to to afford it.
It does not cost billions to introduce a new type. Yes, there is a big overhead, but an order of magnitude or two less than you claim. Ditto for operating it.
You talk about the costs of tracking parts: well, I once (over 20 years ago) helped write an MoD system which did just that (& also costed them, & labour for maintenance, etc). Before that I worked on a similar systems for commercial firms, & a variant for Shorts, which as an aircraft manufacturer had to track individual parts for their entire lifetime. One such system can track multiple types. Shorts (not exactly a big aircraft firm) had all its parts, for all its types in one system, running on a computer which by modern standards was ridiculously slow & short of memory & offline. You could walk down to PC World & buy a much more powerful computer for a few hundred quid now, & with the right software, it could do that job (though I'd rather have a decent networked server with multiple backups). Data entry might be a pain - except that for any current type, the manufacturer should be able to supply files for upload.
The system will tell you when parts need to be ordered. This sort of thing was up & running in the 1970s, & I remember seeing a fairly modest firm based in my home town on a customer list for the first such system I worked on 30 years ago.
Ah, Libya! Trust you to pick perhaps the worst possible example, a country which bought far more weapons in the 1970s & early 1980s than it had people trained to operate, & then quarrelled with enough suppliers to get itself embargoed for many years, & which had a leader whose megalomania was matched only by his administrative & military incompetence. Why not cite Pakistan? With a fraction of the budget, it kept (& keeps) more aircraft operational than Libya ever did. Some of the aircraft it flies now were in storage in Libya, non-operational, for decades, until bought as spares sources by Pakistan, which was surprised to discover that some of 'em were almost in as-new condition, having been stored almost unused.
There are many other examples of forces which would be stunned at the thought that it costs billions to operate a small number of an aircraft type. Why not ask Switzerland, or Finland, or even Sweden or Israel? Do you think that any of those would introduce penny packets of some types (e.g. helicopters) into service if the costs of a new type rather than more of an existing type were so high?
Yes, each additional type increases overheads, but nowhere near as much as you claim - unless the administrative & operational systems are so inefficient that everyone responsible should at best be sacked for gross incompetence, or at worst put on trial for criminal negligence or maybe some more severe charge.