That is not what the numbers are saying. They are the total years the franchise will be considered "useful" to develop. They are not referring to the remaining time in the franchise life which would otherwise decrease by one year each subsequent report year. That "time remaining" went from 5.5 years remaining in 2016 to 4.5 years remaining in 2017 but the total franchise life was consistent in each of those reports for a total of 7 years. The useful lifetime of the Elite franchise however was stated as 8 years in their 2015 report, i.e., it would take them to the end of 2022. They decreased this to 7 years in 2016 which was the same total estimate for 2017 which takes the franchise to the end of 2021. That means they shortened the "expiration date" of the franchise by one full year between 2015 and 2016.
It is literally the opposite of what you are suggesting.
The "expiration date" as you called it has changed once, downward, due to changing circumstances. Changing circumstances can just as easily cause that "expiration date" to change again.
If that change happens to be
upwards, it would be worthwhile to Frontier to continue development past 2021. They're not going to say, "Sorry, can't do it. We
promised to stop in 2021
way back in 2017."
Why would an
upward change happen? There's lots of ways. A certain currently existing space franchise could reach the end of
its useful life, and Frontier receives a sudden influx of new players, all eager to spend their money on this game. A certain space franchise
currently under development could go belly up by then, with similar results. Foviated Rendering VR headsets could hit the market, allowing relatively modest computers to run VR at higher resolutions, injecting new life into Elite from the flood of new VR owners wanting
the killer VR app. Sony could give the green light for Elite using PSVR, or Microsoft gets VR for the XBoxOneX, with similar results. New markets could open, new technologies could be leveraged, all of which would increase Elite's "expiration date."
Frontier, being a for-profit business, is going to be conservative in their estimates for Elite's "expiration date," and isn't going to stake their future on maybe's. But Frontier, being a for-profit business, also isn't going to have plans in place to adapt to circumstances changed.
Which is why, in answer to your question, "Why wouldn't they put those development resources into another franchise that would represent a new source of revenue for shareholders?" my answer is that circumstances changed, and there's still money to be made in Elite's continued development.
But as I also said, if current trends continue, I wouldn't be willing to buy a new LEP for anything more than $60. Because IMO, current trends indicate that the 2015 purchasers will probably break even, and I only buy LEPs if I think they're a bargain,
not a gift card.