I'm not talking about the costs of running your service to support them, but everything else. A part of your free userbase will expect the world for free and start demanding more, and as they outnumber your paid users by so much it can just be a huge distraction. The question is how many of these free users will convert to paying users?

I personally add a free tier to my products because I want to make the tool accessible to hobby and other small projects without a budget, but it's probably not a good business decision.

Something I've been considering: charge some small one-time payment, say $10, for a lifetime 'try-out' plan. Then when they want to upgrade to a subscription you give them that $10 as a discount for their subscription. It may filter those users that will never upgrade anyway.