Well, guys here also argue to not "put all your eggs in one basket", that's the main take. It is too risky to rely on one aircraft model, from one supplier, and sacrifice every other option for it. Even the US with their arsenal of F-35s acknowledged that one trick solution is a no, that's why they even made the F-15EX; to give more options to their airpower capability.
Rafale, even if they are marketed as Omnirole, is still limited in design and cannot perform all missions and requirements in every circumstance. For example, their range is less than F-15 EX, carries less loadout, and has slightly less potent EW than EPAWWS. Then we got into the issue of logistics and politics, France is one country with a smaller industrial capacity than the US, Imagine the waiting list if we go for 100 Rafale, it will probably be in the 2030s when we get the final Rafale delivery. Moreover, we have to rely entirely exclusively on France to provide everything Rafale needs to operate, it is especially important to note because many components are exclusively French and even exclusively for Rafale.
But if we divide our orders into three models from three countries, that will cut the potential length of the acquisition process for the same amount of airframes. Might be logistically complicated, but at least they are all based on western techs anyway, should be less a headache that with Sukhois for example. Then the political aspect might have an even bigger role. F-15EX deal probably carries the most political consideration, as buying them will get us favors with the US, and they do seem to insist on it because of their apparent interest to balance their trade deficit with us. The fact that they offer us a full spec F-15EX is a chance for us to secure US support in the SCS issue, to prevent them from going against our interest in this crucial time.
As for KF-21, this is obviously important because it is a stepping stone for our aircraft industry, and also to preserve our military, diplomatic, and scientific relation with SK. Canceling it out will get us back to zero after all of these years of R&D and cooperation, we cannot waste it. The addition of KF-21, in which we have a part in big or small, is a guarantee that someday we are able to increasingly be less reliant on other countries in maintaining and producing aircraft, very important for the growth of our defense industry. The difference with KF-21 is that we have a stake in it, Rafale is fully French, so does the F-15EX is fully American, these nations have complete control over these planes, but for KF-21, we have partial ownership of it.
So yeah, in each plane there are different stakes. For purely military capability requirements in the near future, Rafale might fulfill it just fine. For political favor and a variety of options, is F-15EX. For industrial, technological, and diplomatic needs, is KF-21. Also, both Rafale and F-15EX could pave the way for KF-21.
That's basically my take on it, you might have a different opinion.