Don't underestimate Mediatek. 5G is years away. The Chinese has their own TD-LTE standards. Pure technology/Performance basis is much easier to catch up than modem technology. The near future will be on "Internet of Things".
On a pure technology/Performance basis, I dont think, Mediatek or others can overtake QCOM anytime soon. The problem is when performance reaches the "enough for most use" point. After that, evenif QCOM improves tech, all the competition will be on price. The good part is QCOM still has breathing time, ie until smartphone totally eclipses the PC.
This may be the beginning of a long fight to survive for Qualcomm. Reminds me of a saying, and I paraphrase: if you are falling from a 100-story building, until you reach the bottom, it feels like flying. Sympathies to Qualcomm.