Tool sites can work, but what usually decides whether it is profitable is where the tool runs. Anything that executes in the browser, converters, calculators, formatters, costs you the same to serve at ten thousand users as at ten. Anything that processes on your server, image conversion, PDF handling, anything calling a paid API, scales the bill with exactly the traffic you were hoping to get. I have watched people build the second kind, finally get the traffic, and find the hosting quietly ate the ad revenue.
The other half of it is that free tools attract people who want one job done and then leave. That traffic monetises badly through display ads and almost never turns into a subscription. The tool sites I have seen make real money were the free thing sitting in front of something people already pay for, not the tool itself. What kind of tool did you have in mind, and would it need to run server side or can it run in the browser?