The problem is Trump has a fragile ego and this is a humiliation to him and his lobbying power. USA already held all the cards in the upcoming trade negotiations as they are huge compared to us, now they hold the cards and have an angry Trump who will want to punish us in the deal to save face and help him in the upcoming election...
At this stage I don't think anybody fully understands the extent of the role Huawei will be given, but it's fair to say that in certain area's they are a lot more advanced than their US counterparts. I don't profess to know much about the technology involved, but I do understand that implementing a national 5G network is crucial to UK business going forward. The UK could lose valuable ground by employing sub standard technology, or by delaying implementation to give the US an opportunity to come up with a better product than they have at the moment. In the long term it could cost us greatly.
Ultimately, there is a lot of business to be had from the UK 5G industry in terms of infrastructure, hardware and software, and I'm sure that US companies will get more than their fair share. But there is more than a degree of protectionism, not to mention bullying, in their behaviour so far and I think Johnson has made the correct decision here.
Trump will have his nose put out of joint for certain, but I'm not sure how much effect this will have on the future trade negotiations. He was always going to play hardball anyway as people on here keep reminding us.