Won’t emergency ventilator production destroy the ventilator business?

Perhaps so, but:

The new ventilators might not last. They might be banned due to low quality after the emergency passes. Some will be damaged by use. Parts might become unavailable.

New companies may enter the ventilator business. We still have the ventilator business, even if the companies change.

The demand might remain high. The disease could stick around. This isn't the first respiratory disease to start spreading right next to a Chinese biohazard lab, and likely won't be the last.

A great deal of innovation will occur by smart minds released from the box and they will publish many design innovations which will act to bypass the vertical price controlled and patented etc towers of the old makers. All this will be released via open source methods anda general constructive frenzy will occur = more and better as well as cheaper ventilators. This is good, very good.

It's one of many very sophisticated product lines. The companies will shift their focus and still innovate. From what I've read largest manufacturer is https://en.wikipedia.org/wiki/Dr%C3%A4gerwerk

Who’s going to need to buy a ventilator after this is over? There might be a small market for technical advancements.

hopefully