1. It's always been technically and administratively possible, you're just the first genius to overcome those hurdles. All the combined efforts of thousand developers at a FAANG found it impossible, but against all odds, you succeeded where they failed. Congratulations if that's you, but don't bet the farm on that being the result before you start.
2. Technological developments have only just now made it possible. Maybe your idea requires resource-heavy neural networks on the users' mobile phone, and current high-end models have just become capable of that. If tech has only now made it possible, you also have to be the first of millions of developers to realize that and get to market. Good luck, this is how most of the startup ecosystem has worked for a long time.
3. It's currently illegal, and you're just going to hope no one stops you. This is a big administrative problem, and maybe all the efforts of a big company couldn't change the law and make their idea work when fighting eg. the hotel or taxi lobbies, but if you side-step all that and just do it anyway, well, you might see all your hard work disappear in a courtroom, or you might succeed.
I'm working on a product where google is my competitor and the thousands of super genius engineers utterly failed to achieve what we did.
Just because the big company of the day didn't do it, doesn't mean it can't be done.
I am using a service called Invoicely for recurring invoices in my small business. One day I randomly googled for some of its features and found a post of a guy who claimed (and I believe him) that Invoicely is a blatant screen-by-screen clone of their own SaaS.
I felt really bad for that guy, and after some time I thought that it's a good idea to switch to them. The problem: their website name is so difficult to remember, I never got around to actually opening an account there.
Some shifty guys literally copied an existing service up to their Javascript and CSS, and improved it dramatically just by giving it a name I can remember.
You mean, like gas stations?
I used vk.com a lot in the past and in terms of UI it was much more convenient for me than Facebook.
Same ideas, different implementations.
I'm talking about people who just rip-off someone else's design with adding anything original.
-you didn't look hard enough
-you are a genius who spotted an untapped demand (very unlikely)
-no-one wants it (very likely)
Also, remember that the main competitor to a lot of software is Excel and post-it notes.
That said, the more competitors the harder it is to get noticed. Ideally pick something where you have got a reasonable chance of getting on page 1 or 2 of Google.