Paying for cup games or playoffs is standard practice the world over.
Think of it another way - as we debate our club being skint --- the club can budget or at least be aware of the fact that they will host 16 home league games. These have costs that need to be budgeted for when we set ticket prices. Costs such as heat, light, stewarding, policing, etc etc etc. We cannot know before the season how many cup games we may be involved in, or if we will be in the playoffs. If we did - and having these in the package was actually allowed - then we would have to factor in that cost into the overall season ticket price. You would not "pay" for these games when they came along as you already did so ......
I was the same over here in 2016 and 2017 when Toronto got to the MLS Cup Final .... cost me about $300 for my season ticket, then as the playoffs came along it cost me the same again over about 4 or 5 games in our run to the final each year. Same applied when the club were in the Champions League over here. We actually got the first game 'included' in the ticket price but had to pay for all others in our run to the final of that competition too ! My $300 season suddenly became about $900 !
Personally, I think crowds are down because people have so many other things to spend their ever decreasing discretionary cash on, and the football authorities have, over the years, made the sport less enjoyable and killed the atmosphere to a point where going to a game is not high priority for those outside of the diehards.
There are probably things the club can do better, but its not their fault that a typical football game nowadays involves 'customers' turning up to a stadium to be treated like potential criminals for a disproportionate sum of money that has to sustain ever spiralling costs, to be fed on over-priced crap, while watching a game that is inevitably of a lower standard than we have been used to because the Scottish game just cant compete even with the lowest tiers in England because of TV money.
I dont know the answer, no clue at all, but as much as the club can do, the SFA and SPFL need to try and figure something out for everyone's sakes !