So? Do it anyway. Competition stimulates demand and the fact you've got long-lived competitors indicates there's demand already. By coincidence I was literally about to sign up for Pingdom, but I'm going to try these guys out instead. If this had been your service, you'd have just had a new customer right now.
Your success will depend on the quality and efficacy of your marketing.
So was I - about 10 years ago. Registered the domain name, started working on the code and then realized there were 100 people doing it already. I mean, seriously, the code to do this is trivial to write.
So I mothballed it for a couple months and then got an unsolicited offer from a guy in Israel to buy the domain for $1,000. Easiest $990 I ever made.
It's a fun way to learn about a good variety of things; http and other protocols, distributed architecture, how poor most sites' uptime truly is...
I built https://servercheck.in/ for that reason, and because I wanted cheap 'real' SMS notifications (from a consistent phone number) so I could set a special ringtone on my phone when one of my servers goes down.
