Your whole service could end up being one giant backdoor for every application that uses it, putting millions, even billions, of people at risk. Imagine hospitals using software from your system that was never signed. Nuclear facilities, oil pipelines, water treatment plants. If you think 'nobody would be that stupid to use unsigned software there', think again. Some organizations would probably ban the use of apps packaged by your service if it wasn't clear that they had been signed by the authors.
If you make the process seamless enough, people will be fine with signing their builds. And there's plenty of examples of supply chain attacks you can wave around for encouragement. You can even promote this as a core reason to use your service, as you can make it easy for them to securely distribute their software.