MI5 never passed Huawei telecoms products as "fundamentally secure". Their claim was much more limited. The UK government could still securely share intelligence with the USA even if the civilian telecoms infrastructure used Huawei equipment. Presumably government and military networks would still use other more trusted suppliers.

https://news.sky.com/story/mi5-chief-shrugs-off-us-warnings-...

Regardless of security issues, allowing an adversary and competitor state such as China to control your critical infrastructure is clearly a stupid idea. This is a strategic issue which transcends economics.