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.
I understand they're slightly different from one another, and I suppose its possible all the commenters here went through each and every one of these options and found nothing useable but it seems unlikely to me, and similarly unlikely the +1 one would make in this pool would be any sort of revolutionary "game changer" VCs would mob onto. The space seems well-established or dare I say it "saturated".
Also, “exact replacement for killed product” is a realy good advertising pitch to differentiate from those thousand other versions. It worked for The Old Reader, and later Feedly, when Google Reader was killed.
You're right that the idea is the easy part. Still, there are a number of TeamViewer clones, ranging from terrible to pretty good. They may not have the same brand recognition but TeamViewer is quite expensive and not everyone is willing to pay top dollar.