The billions of dollars that the Chinese Government will invest in order to compensate for the sanctions might have the unintended side effects of actually making performance of Huawei gear more efficient. The jump from FPGA to ASIC is pretty big as far as efficiency, and being able to have it financed for free to defend against US sanctions is pretty sweet.
They also happen to have stockpiled enough FPGAs to buy time for the transition.
Building a competitive ASIC will also be significantly more challenging now that Huawei has been cut off from all the major EDA companies.
Finally, I assume that Xilinx is well into development on future product lines based on newer processes. It would be extremely difficult for Huawei to bootstrap their way to commercially competitive FPGAs.
They can make their own FPGAs, true. These FPGAs will not be commercially competitive with Xilinx and Intel, so development costs won't be defrayed by commercial sales. IP laws may effectively prevent their sale in Western markets, anyway. And base stations made with these FPGAs won't be as commercially competitive, either (they'll be larger, more power hungry, produce more heat, etc.).
I don't know enough about 5G or base stations to say with confidence what other alternatives they might have. But I'm sure that they will be sub-optimal, at the very least imposing billions of USD in costs to Huawei/PLA/CPC.