If you purchase software that your company relies on, it belongs to you. Regardless if that third-party disappears. You still have use of the software until you can determine if you need to replace it or if it'll still work out for your needs.
Now renting the software as a service is a different matter. If the third-party goes under, well then you have problems. Problems you may have to solve immediately.
As an individual who is trying to build something, a disappearing third-party may not be detrimental if you still have the software at hand. You would not be locked down by that third-party's SaaS.