Even if US companies only had the US market, they'd be massive — Google gets ~50% of its revenue from the US alone, Amazon over 60% from North America, and most other Big Tech is in that same range. The US market by itself is plenty huge. And the EU provides 20-30% of US Big Tech revenue. Even losing all of that (very unlikely even under protectionist policies), US tech companies would be doing well, with 70-80% of their current revenue.

Sure, full-blown protectionism everywhere would make the world including the US poorer (less specialization, less division of labor), but it would also harm EU exports, as the US is the EU's biggest importer, and moreover it wouldn't change the factors behind the growing US-EU gap. US-EU trade policies with each other are basically the same. The difference is internal, and mostly comes down to the US just not sabotaging the private sector as much.