If you don't want to pay, then host Sentry on your server, the company behind Sentry doesn't lose anything. In fact they can still benefit from you since you became a part of the community. These people usually advocate Sentry, report bugs etc.
However you need to differentiate your product from your competitors and error-reporting is not actually quite a hard problem, I know many companies that still uses e-mails (send email with stack-trace directly from the app) for error-reporting. Sentry has a unique approach here, it's an advanced error-reporting tool and tries to build a community around Sentry with open-source.
If you're a developer who needs error-reporting for your new fancy app, instead of your custom solutions (which are actually workarounds) install Sentry easily and get an advanced error-reporting application for free. Eventually when you work in a enterprise or start a company, you will want to stick with Sentry since you're familiar with it and don't want to take the responsibility to host Sentry so you will switch to SaaS solution.
I'm not actually related with company behind Sentry, these are just my thoughts. :)
It would be crazy business to throw our biggest competitive advantage over board. It's not that we do not know why people use us :)
I will be incredibly happy to be proven wrong on my fears.
It is the companies that raise crazy amounts of capital over many rounds who end up at the whim of boards and investors.
Also, hi Nik, been awhile!