Huawei’s, MediaTek’s, and Intel’s 5G modems were nowhere near as good as Qualcomm’s - most likely because of Qualcomm’s patents. If MediaTek etc. was as good (or better) than Qualcomm, that’s what Apple would be using. The only reason Apple is using Qualcomm’s modems is because they are the best out there.
RF is really, really hard. Unlike CPUs/GPUs that largely live in a digital-only realm (logic gates in a CPU/GPU are either on or off - I.e. allow current or don’t), RF has to bridge the divide b/w the analog and digital world. Not only that, but with something like a cellular modem, the amount of variables you need to account for are astronomical when compared to a CPU/GPU. With mmWave 5G (which has been a flop, whereas regular 5G has very much been a success - the two are often conflated but are largely separate from each other), something as simple as your hand can block the signal.
I have no doubts Apple could develop a perfectly fine 5G modem (or a BT or WiFi modem), but it’s really hard to develop something that is great. Apple isn’t going to switch from Qualcomm for an in-house design that is simply fine. If/when they make a switch, it will be because whatever alternative they choose (be it an in-house design or a chip from another company) will be better than what Qualcomm has to offer.