There is such time as BYOD. As things stand. GSM phones working with ATT and Tmobile. By next year, Xiaomi might be in the US to compete.
Xiaomi likely won't be in the US anytime soon, regardless of what they say. The big 4 telecoms run everything in the US and Xiaomi has no experience dealing with telecoms. Nor do they have the engineers to do what is required to get their phones working with any carrier at this time. Xiaomi only has 14,000 employees globally, most of which is retail in China and India.
Then we get into the IP issue. Xiaomi is the largest copycat of Samsung and Apple hardware and software. And while they have spent a lot of money on patents from Microsoft and Nokia, it's not nearly enough to take on companies like Apple and Samsung that will be waiting to slap Xiaomi with a billion dollar lawsuit the second they step foot in IP friendly countries.
Lastly, we are right on the cusp of bringing 5G into the US. This is going to require new hardware on all new phones, not to mention the testing by the carriers and OEM's to get everything working properly to utilize 5G. It makes no sense to break into a market with phones that will be DOA the second they begin selling.
I'm not saying it is impossible, I just doubt we will see Xiaomi in the US prior to 2021.