So the only really legitimate issue with the SSD appearing in the game (since a 'sliding scale' obviously allows for its size via any kind of curve-plot) is that it is "supposed to be so powerful that it could not meaningfully fight with the number of ships you could bring to a 300-point match". And I think we can grant that - the SSD would have to be in 'epic' scale (say, 1000-pt-games) to begin with. It IS big!

'Even then', some argue, 'it would not be powerful ENOUGH at 1000 points, all on its own, to out-fight the dozens of Star Destroyers it should be able to'.
And that, a seemingly fair argument, will be what we talk about, today!
Looking at the concern, I see two fairly high-level approaches to addressing it. The first theory is that the EU probably overstated its capabilities quite a lot, and so it actually does fit within a 1000-pt game without issue. Second theory is that we've got the scale considerations far more wrong than we realize. In order, then!
- The EU source most commonly cited is West End Game's RPG (originally, although much of what it proposes made its way into "Legends" canon and can now be found on Wookiepedia). This gives us a ship with a thousand batteries of various kinds (turbolaser, ion cannons, missiles, etc). A pretty big step up from the Victory-II, which WEG's tells us has 50 batteries (turbolaser, ion cannons, etc). One ship with 1000 = 20 ships with 50, after all, so even looking at that rough estimate, and knowing that a Victory-II costs 85 points, it tells us you couldn't fit even a single SSD into a game of less than 1700 points.
HOWEVER, this proceeds on the assumption that a Super Star Destroyer is approximately equal to 20 Victory-II-class Star Destroyers. Is it? According to the EU, YESANDTHENSOME. According to 'canon'? Well...who knows... We know it's a "command ship", so it's theoretically possible it carriers no offensive weapons at all (unlikely, granted, but modern USN command ships like the Blue Ridge-class ARE practically unarmed in comparison - and we never did see the thing fire a shot on-screen), and just limited to anti-fighter defensive batteries. As noted - unlikely, but nothing makes that impossible. The real point is that there is so little 'canon' information on the ship OUT there that drawing conclusions from WEG's RPG cannot be considered conclusive. The ship could be anywhere from 0x the capability of a Victory-II to 20x its power or 1000x its capabilities. Slotting in at 10x would make it an 850 point ship - steep, but certainly possibly to include in a 1000 pt game.
- We might just be flat-out wrong about the "scale" of the game! Or, more specifically, what the scale represents.
Consider, we've seen so far that the game has a mini for a "squadron" of fighters...
...I've never seen any branch of any armed service consider three fighters "a squadron". The normal number is 10-14 (depending on the service). Star Wars has traditionally used 12-fighter squadrons (canonical reference: a few dozen episodes of 'Clone Wars'). It's obvious that these three miniatures actually mean considerably more ships than we see - potentially all 12 of a typical 'squadron'.
But why do we assume that is true of only the fighters, where one fighter 'mini' represents 4 'actual ships'? When we first saw the ship point costs, I think many were surprised...
...TWO CR-90s are worth MORE than a VICTORY-II-CLASS-STAR-DESTROYER?!
It seemed ridiculous! But...maybe it is. Perhaps the 'large' ship counters represent multiple units as well.
What if each CR-90 mini represented not a single vessel, but a group of three CR-90s flying in very close formation, operating as one unit? And even the VSD, itself, representing a pair of such ships? Suddenly two CR-90 minis on the table (representing 6 ships "in universe") seems a bit more believably scaled against a single VSD-II mini (representing 2 ships). This doesn't even necessarily conflict with the concept of "Commanders" or "upgrades" for the ships, as it would be entirely reasonably for any given flotilla (represented by that one mini) to feature one 'lead' ship - which gets the senior commander and upgrades - and one or two 'wingmen' that are of the same class but simply there to echo the actions of the main ship.
And we could then establish the Super Star Destroyer as the baseline unit of the game, in being the only unit where one miniature = literally one vessel.
Suddenly, *boom*, all the scale problems in the game are fixed. ALL of them - that the CR90 mini is 'so large' next to the Victory-II is no problem...it's representing more than ships than the Victory-II does. That the point value would imply a Victory-II is not even as valuable as two CR90s...again, not a problem, it's not 1 vs 2, but 2 vs 6 (or whatever). etc.
So, IMHO, at least two good and reasonable options to explain the SSD being added to the game via an 'epic' scale.
IMHO, that the game was announced with a 'sliding scale' in the first place made it obvious FFG's intent was to include the SSD - but I think these present at least a couple ways of thinking about things that explain how they'd be able to do it, too!