I have had many month to month tenants in multiple properties over the last 30 years, most worked out very well.
A lease obligates you more than them. People break leases all the time, and it’s a real hassle to get a judgment and/or collect.
A lease is not a guarantee that a tenant will stay there.. It IS a guarantee that it’s a lot harder for a landlord to get rid of a crappy tenant when they are paying their rent.
If someone has a lease and wants to break it, do you really want them in your property as disgruntled occupants ?
Being a landlord comes with huge risks and obligations that most people seem unaware of, until something goes wrong.
If you feel better with a lease, you can raise the rent with 30 days notice, and offer a lower monthly rate, or keep it the same as now if they sign a lease.
I once had a month to month tenant stay for 13 years and the only reason she left was because I sold the house.
Month to month does have to be a bad thing.