Losing to the Sukhoi in a non-realistic training scenario is surely no indication of how the F-15 would perform in combat against them. Also, although the F-15 lost to the Indian Sukhois (remember, the F-15 also lost to the Sea Harrier FRS.1 in exercises in the 1980s), the opportunity for F-15 drivers to see how their potential adversary was likely to perform in combat first hand would have been extremely valuable to them.
Regarding the F-35 and its relevancy, it is most certainly relevant. I've never thought much of that old chestnut that states the F-35, the Eurofighter and the F-22 were no longer relevant because the Cold War has ended for the following reason. A country's armed forces is like an insurance policy; you sure as hell hope you don't have to use it, but its there just in case you need it. Like insurance policies, when situations change and what went before becomes obsolete, the old stuff needs to be replaced and the new stuff designed to meet the requirements of the new situation. The B-1 is a classic example, it was designed as a high altitude supersonic bomber, but because philosophies changed, so did it. Its combat record has proven its worth.
From an engineering persepctive, aircraft design is incorporating more and more avionics and non metallic structures that engineers need to become trained in new disciplines; black handers are learning avionics and composite repair, and this isn't just happening in military aircraft either. Therefore, the justification for maintaining the development of Cold War era combat programmes is advancing technology that came about as a result of the Cold War. Even if the first marks of the F-35 prove to be troublesome, the US are going to throw money at the aircraft until it gets better, as they did with the F-111 and B-1. It has to work; they've put some much into it and so much is riding on its success.
I don't know much about the performance of modern combat aircraft, but regarding the F-35 being inferior in capability to the F-111 its replacing, show me an aircraft in service today that has the performance and capability of the F-111. There isn't. It was unmatched and superbly suited to the Australian environment. It's highly unlikely that an aircraft that could match or outperform the F-111 will be developed within the expected lifespan of the F-35 in RAAF service.