IMO it's not just better scan lines, but even better CRT effects than what the MiSTer itself does. And being able to get 720p HDMI from a PS3 or X360 and outputting to 4K with CRT effects will give a much better image than the old HDTV CRTs of the time gave.
Most TV scaling from 1080p to 4K is low latency, but 720p and below can be higher. Now we can have integer scaling that should only be a frame of lag.
And yes the price is very high, but people were dropping a bunch of money on framemeisters years ago. And with inflation, the price on a Tink 4K isn't that really any higher. There's always the Tink 5x, and OSSC Pro for 1/2 the price that will give most of the features at up to 1440p. But the Tink 4K feels like throw whatever you want at it, and you'll get the best possible image back, even beating out 5 digit professional hardware.