Check out the Suggestions forum. There's a constant stream of requests to automate/shorten/simplify activities which people find tedious/repetitive/boring. If exploration had always been the FSS I guarantee that people would have been requesting functionality similar to the ADS. Give it 6 months and they'll be there, from people who never experienced the ADS.
Not really possible to guarantee a theoretical but I agree up to a point. While the Suggestions forum carries some extreme entitlement, I don't doubt that requests to automate some of the FSS would have come had it been there from day one, just as they have come with this implementation. But I struggle to imagine many people would have put in a request that was basically "Let us see all of the planetary types and the approximate system geometry at once, but force us to fly to each body if we want to log them for discovery." And if they had, I can't imagine it would have met with the same universal approval as that granted to the ADS by those who miss it the most.
The rationale for not allowing both to co-exist just isn't ... well ... rational
FD really ought to have done what was suggested during beta, and populate the FSS spectrum onto the HUD. It wouldn't solve the issues of everyone concerned, not even of everyone in this forum, but it might have offered a half-way solution that demonstrated some willingness to move the goalposts. Having to slow down and engage a different mode in order to see what's in the system is undeniably irritating even to those who generally like the new mechanics.
The scanner is listed in the modules tab, but as it doesn't use any power, it can't be turned off.
Alas. I wonder how much effort would be required on FD's part to open up zero-draw modules for disabling/re-enabling, and whether it would cause issues elsewhere. It seems as though a useful option is being denied simply because of an interface choice.
Of course it may be that there is literally no code for a non-functioning scanner precisely because it can't be turned off, which would make it a more difficult proposition from a programming point of view.
You don't have to wait to throttle down and come to a stop before opening the FSS, it will open up as you're throttling down.
Very true. With the right software you could even bind a macro to do both at the same time.
Which further raises the question of why you have to throttle down at all. The FSS will still open even if you're hurtling along at ludicrous speed, as long as you've zeroed the throttle. The ability to engage the FSS while travelling at high cruise and/or charging the FSD would again remove a lot of the frustration with the new mechanics. Certainly being able to spot a dull and useless system without having to slow down would help. It wouldn't be a million miles from how many people used the System Map in combination with the ADS.