It's not the EMUI updates you have o worry about. EMUI is a Huawei product, and they can keep releasing new versions for decades. It's Android that they won't be able to release after - probably - September. (Google can still ship the September update o them at the beginning of August, which is the normal schedule.) After that, the "Android" part of the firmware will have to be replaced by something else and, since ARM (the System-on-Chip in an Android phone) is part of what they can no longer buy, they'll have to come up with their own silicon (or, at least, Chinese silicon) to run their own system. (MIPS is going open source, but the last version of Android ported to MIPS was around 4.0.)
So current phones would probably use Hongmeng (presumably running in Linux, the way Android does), but coming out with new phones would be a problem. They could come out with a phone using a Kirin SoC and more storage, more RAM, any new open source additions (like new Bluetooth protocols or new Wifi protocols), but they wouldn't be able to come out with phones compatible with US 5G standards. And, even though some Chinese company has bought a 4nm silicon foundry, going from the foundry to an SoC is going to take years.
But the current phone will still work and it's still going to run Pie (I don't see it running Q). And if you don't have Google apps on it, you can still go to
GApps and install them ... so current owners have very little to worry about. (And from what I see in the Q beta, it's just more of the same - nothing exciting except for continued security patches (which will probably appear in AOSP, so Huawei can still apply them as security updates.)