I believe you need to take certain units into account when building your list.
These units are:
Space Marines
Falcons
TMCs
Guardsmen
In my experience these are the units that you need to be able take care of.
Land raiders, genestealers, and terminators are often added to this list but are not needed. Your list's ability to deal with these things should be a side effect, these units do not tend to dominate games where they are unkillable (especially the land raider).
You need to work out the probability distribution of your units when you shoot at those 4 units. For each of those 4 units, you need at least one unit that can handle the job.
Well put. You expressed very succinctly a system I've been using for years. Of course in v3 the other big category was Smoked Rhino, not Falcon. (And for the current prep I'm doing, I include Termis.
GS are largely redundant. Monolith is something I include in non-Tau army design, because it CAN be dominating and other armies don't have the instant anti-Monolith solution that we do.)
In the overall feel of the list, I have chosen to say I prefer broadsides shooting at falcons, even though they cost more in points, and even more in protection and gravity, simply because I price risk very high.
Gravity in an army list is not an inherently bad thing, particularly not in an army list like Nikkenryu Tau where fire priority from a mix of mobile and static elements is how you deal with threats. That gravity effect can work against you
if you let it, but mostly works against your enemy in my experience.
(The point at which it begins to work against you is the point when you forget that your Broadsides are also expendable.) I think you're overstating their survivability. There's too much out there in army lists that can and will get to your Broadsides if you make them a tasty enough target - that "good gravity" in a too-much sense - otherwise an even more static design would work better and you wouldn't be worried about the "bad gravity" effect. You're talking about them the way I used to when I ran 6 of them and screened them behind 36
FCW. Now
that was gravity, and also survivability - and the Broadsides were 450 points my opponent would never claim. They're not that butch now as far as I can see.
I don't think we're that far apart on theory of list design and tactics. Sounds like a method of expression problem. That's cool - it took me a long time to get people to understand what the hell I was talking about 5+ years ago when I said "Tau CAN win, and here's how". I am totally ready to accept that I'm maybe just not "up" on the current jargon - maybe all this stuff is retread for you and I'm just not hearing you when you say it.
You can't buy only units that make their points back - those units don't exist in enough balance and you end up with stupidly blind army lists - but you can avoid the most wretched purchases and useless wargear point frittering.
You can't guarantee that you brought enough of the right gun for every army list - but you can guarantee that you brought the right unit to deal with each target class in an effective and not-terribly-inefficient manner.
-JTS