IMO, the choice for F15 has both polictical and practical
experience.
In terms of political, it makes sense because we need to get into
bed with the Americans, we can make use of their training
facilities, instructors, etc. Imagine you buy Typhoon and where you
gonna train? Its only in US that you can fly your F15 with F16,
coordination with our AWS aircraft and perhaps vs Apache, and all
the hardware that we might have at the Prairie Training
Ground.
Its crucial that we can formulate battle plans incorporating the
different a/c that we have and then actually do it in the training
gnd. I don't think US is gonna let you fly the Typhoon over and
base them at the training ground ? Also, flying the F16s over may
not be a good idea as well.
I am not sure, but I suppose cross-training support staff will be
easier to maintain and arm the F16 & F15. In the sense, weapons
can be developed with the two aircarfts in mind / system.
Lastly as many have pointed out, purchasing the F15 can be a
stepping stone to purchasing the future generation aircrafts that
may come off the line.
I think perhaps one of the most crucial reason is its trackrecord.
The F15 is available, its playing, its taken part in combat and it
has shot down planes without a single loss. That is the testament
you need to pick the aircraft. The other planes are not yet flying
or not that "well-known". Without doubt, the F15 is considered old
but it is only a stop-gap. I think RSAF is gonna purchase a newer
batch to replace its F5 & early F16s soon.