As a French founder, am I authorized to do a business trip in the US (meet local customers, pitch product features, invite prospects at a restaurant) with a simple ESTA?
Yes but you need to be careful because the line between business development/marketing and work - at least as perceived by CBP officers - can get blurred. I am assuming that the only company here is your company in France, and not in the U.S.
If they are really concerned or suspicious, they will sometimes ask to see your phone or computer. But their analysis is what in fact the individual will be doing in the U.S. - that is, will he or she be doing productive work - and who is the primary beneficiary of the individual's activities.