Theoretically you are right. However the Kickstarter began in 2016. The HDR41Q HDMI repeater from MDS was out in 2017 : at that time this piece of hardware was top-notch. The APM89L audio decoder though was from 2014.
Should we deduce that we are good for another 3 years of upgrades or less if the future deliveries arrive later...?
Theoretically you are right. However the Kickstarter began in 2016. The HDR41Q HDMI repeater from MDS was out in 2017 : at that time this piece of hardware was top-notch. The APM89L audio decoder though was from 2014.
Should we deduce that we are good for another 3 years of upgrades or less if the future deliveries arrive later...?
The way I see it, it is very difficult if not impossible to make any such deductions with any accuracy. The HDMI repeaters, as you correctly point out, are upgraded with a fair amount of frequency. This, however, doesn't necessarily mean that the next upgrade will necessarily have any significant effect on the A16's functioning. Once SR has finally implemented all of the promised functions then that should, from a software standpoint, put them on track to focus on doing periodic firmware updates. That is to say, the A16 may become, like most pieces of kit, a closed hardware system.
The biggest hardware question I see is if and when they will upgrade to further HDMI standards. As
@Got the Shakes points out, it would be a home run for backers/owners if the HDMI boards were to be upgraded at the same time as the rest of the hardware. However, this does not look to be in SR's plan, at least for now. Beyond that, the big issue, at least for me, is the clearly stated possibility that firmware updates down the road may not be equally functional in different hardware configurations of the A16.
At the same time, the glass is more than half full even for A16s shipped prior to April 2020 as SR has already produced a revolutionary product with amazing capabilities not available from any other company in the world for any amount of money.
Before I make my next statement, consider that, short of headphones(e.g., my Hifiman HE1000 which is now a HE1000 v2), none of the other components in my audio system have hardware and software updates available from their manufacturers; if I want to upgrade them then I have to do so by purchasing an entirely new and different piece of equipment. I know that I shouldn't admit this, particularly given all of the promised features yet to implemented, but I could live very happily for the rest of my life with the A16 as it exists in March 2020. That, of course, doesn't mean that the feature sets to be made available in future hardware and software updates is not something that I would welcome and, up to a point, be willing to pay money for.