Sure it is possible to bring up a virtual instance and link it with a s3fs filesystem, we also did this many years ago, and learnt our lesson! Managing and monitoring such an instance became a serious overhead, and, there are no equivalent s3fs fuse plugins to Azure or GCS. There are also considerations of high-availability with SFTP and FTP/S, file sync between instances, and dealing with sftp if on a Windows environment. Then load-balancing...
Our service is fully managed, highly available, multi-region and multi-az. We support, maintain, monitor 24/7 with regular feature upgrades. We offer a user jail for multiple users to the same bucket out of the box, even on our basic plan, that is just the beginning.
Our job is to worry about this, you've got bigger fish to fry!