To be fair, this may never become an issue.
That’s because enough weapons are likely to be added alongside troops, as Kingdoms exceed 17 stars.
As an aside, I personally don’t buy the logic that weapons previously available for A$8 should not be made available by other means. Many people are willing to pay to have things now rather than later. I think the current situation is incredibly confusing, and not even close to what we were expecting. That’s a real problem. (Why confusing? Because it’s very hard to believe the “money only” list of weapons will not change in the future; because it doesn’t make sense.)
Back to Koromac’s point: let’s take an example. Ghulvania looks like the kingdom most likely to get “weapon-blocked” at and above 20 Star Power. It has 21 troops but no faction, and only 7 weapons: 1 Doomed, 1 Class, 1 craftable Event weapon, 1 non-craftable Event Epic, 1 Xathenos craftable, and 2 others. So only 3 of those are obtainable by play, and 1 is cash-only. A Faction will take us to 25 troops and 4+3+$1 weapons.
To reach 20 Stars, Ghulvania would need another 4 troops and 1 (non-cash) weapon. Troops are normally added either as individual Mythics/Legendaries, or in groups of 1-3 troops during a single week (Glory, Invasion/Raid, Bounty). One weapon is normally also added with any Invasion/Raid troop.
So the most likely (quick) scenario for reaching 20 Stars would be one Mythic/Legendary, then one week with 3 troops. That would come with an extra weapon, so we’d be fine. Worst case would be a Mythic plus a Legendary, plus a Glory troop and a Bounty troop during Guild Wars (so no extra weapon). That would leave most people 1 weapon short of 20 stars. But that really is the worst case.
So basically, if the devs are careful, no-one should be weapon-blocked to 20 stars. Simply because weapons are usually added along with troops.
However, looking beyond 20 stars, this issue does indeed become more likely. The current pattern, above 16 Stars, is 1 extra troop per Power Level, and 2 extra weapons maxed per 2 Power Levels. We’ve seen that weapons are added to a Kingdom at roughly a third to a fifth of the rate of troops – and definitely not the 1:1 required by that pattern!
So that is the long-term danger. But of course, we don’t know what will actually happen beyond 20 Stars (if anything).