The calculus has changed; to put it simply. People don't like sitting around idly, and if there is the risk the official supply chain infrastructure can't keep up, people will rise to try to actually solve a problem.
Without the threat of an overwhelmed healthcare system, why worry about a DIY ventilator? It would comfortably register as SEP (Someone Else's Problem) on most people's radar given the complexity of the rat's nest of regulation around the industry. With the crisis being what it is, however, you suddenly have a very interesting optimization problem, with very high stakes.
I can't fault folks for at least trying; and at the end of the day, who knows, someone may make something better/more efficient/cheaper/reliable/good enough to help. You'll never know if you don't try.
Just remember: do your research, learn why the current state of the art is what it is first, then make sure you can justify any design alteration you make in your particular implementation, then consult a regulator. They are there to either help or to specifically be part of whether or not your design is good enough given the logistical shape of the field.
That doesn't mean everyone go spam the FDA at once, mind. You don't want to DDoS the regulatory pipeline. If you're serious about it though, and willing to jump through the hoops in hope of being able to provide an alternative just in case; do so. Note it doesn't hurt to put a little more strain on the regulatory infrastructure as well. It helps the regulator to justify innovating how they can do their job more efficiently to!
Then, once you have regulator buy-in, go to your investors, and make the case for getting some mass production spun up, or figure out whether your job is done in the sense you're more of a design person, and less of an implementer, and get your design to those who can run with it.
This is the magic of the market at work! Not some arbitrage scheme, but an honest to God shift toward creating a greater supply capability to fill increased demand.