I think you are wrongly assuming that there are no new fans discovering the original series. Studios are still making Star Trek movies, and every time one comes out, a new batch of kids potentially rediscovers the original series. These movies aren't being watch only by us old fans.
My teenage daughters went back and watch almost all of the old Dr Who episodes because the new series has captured a new audience. I think Trek continues to do the same thing.
I believe the original series may be of interest to younger fans, but I still think that they will view it more as a curiosity, but consider that even the younger fans who fall in love with series, there will only be a percentage that will want screen accurate props.
Look, my point is not to dissuade anyone from making kits, I'm simply saying that it is my opinion that within the next 5 to 10 years, the demand for certain types of licensed merchandise based on the original Trek TV series will start to wane because the fan base who desires screen-accurate props are older and will already have what they want -- and the market for making yet another screen-accurate version of the communicator, phaser or tricorder will not warrant the costs to produce them on mass scale.
But rather than having me continue to rationalize my opinion, perhaps those who feel differently can explain to me why, 5 to 10 years from now, you think there will continue to be a huge demand for screen accurate props from the original series. I ask because I am sincerely curious why you would think this? Do you truly think the original TV series will continue to hold such interest for future generations that they'll be seeking out such merchandise?
Again, this is simply my opinion -- if you had asked me 20 years ago if we would have multiple companies making licensed figures and models of various starships etc., along with toy versions and high end replicas of the props, I would have laughed and said absolutely not...(and i would have been dead wrong)...but now that all that merchandise has been put out there, I find it hard to believe that the original series will continue to generate such interest another 10 years down the road...especially enough to create a renewed interest in the props.
Disagree? Cool -- tell me why you feel differently? Do you think they'll keep making TOS prop replicas 20 years from now? 30? 40? 50? When do you think it will stop?