The 21 billion euro valuation of Mistral AI is a strong signal for that specific company, but its likelihood of directly determining the broader regulatory viability of the AI Act is relatively low, probably p<0.15.
Investor appetite for one company is not a universal endorsement of an entire regulatory framework, as the costs for a large, well-funded entity are proportionally much smaller than for a smaller startup.
For example, a Belgian startup with a 500k euro seed round will find compliance costs represent a vastly larger percentage of their budget, making the AI Act a very different proposition for them.