I mean that you can make a Screenhero clone in approximately the same amount of time that it took to write it originally. Should be a bit less because straight path (cloning) is shorter than twisty one (things that Screenhero did that did not end up in the final product).

Note that by the time of Slack acquisition Screenhero was approaching v2.0, they had product out far longer than that. So it's not unreasonable to say you could do a v1 of similar product in a year.

Both idea and execution are hard. Cloning existing idea removes one hard part, therefore increasing the chances of success. Look at Excel, Word or Baidu or Sina Weibo.

As to "thousands of other versions of this": you would have to point to at least one. It's hard to argue against imaginary products.

Based on this thread for many people Screenhero has no acceptable substitute.