I have to disagree with a lot of people on this thread: the title may not matter now, but it absolutely
will matter later on. When it's a five-person company, everybody does everything and people can directly measure each other's contribution. Totally different story when the company is 50+ people, opinions of others' contributions are based on secondhand accounts and recognition, etc.
Titles set expectations about a person's level of contribution, input, and ultimately, control over the organization. You have to be very careful about this.
My advice is to optimize for the success of the organization. If giving someone a founder title is what it takes to keep them on, and they're a vital part of the team, then give them the title, but only if it doesn't set a bad precedent, and other team members who don't get the title/recognition won't be (overly) demoralized.
For the engineers out there: this "organizational debt" stuff is like tech debt; sometimes you make short-term decisions because they're the right thing, and you have to do it, but eventually, undoing them will require a lot of hard conversations, corrections, and people might ultimately leave or worse, speak negatively to the press or other outsiders about it.