Why? Because NATO will follow France within days.
French Missiles fired into Russia will most likely be redirected Ukraine land areas with Russian troops. Putting EU Nation soldiers into Ukraine opens them up to direct conflict.
France cannot go the Russian or Belarus boarder as a red line either.
Look for NATO SecGen to reiterate no single Nation sending its soldiers into Ukraine will trigger Art 5 or otherwise obligate all of NATO to go to war with Russia. NATO will have be more clear than using words like “obligate” and renounce going to war with Russia or things could get messy.
The NATO Treaty had to be approved by the Senate. Any revisions to Art. 5 will likewise require Senate ratification. If the US sits this bastardized application of the treaty out, do you really think the Euroweenies would go it alone?