You're thinking too narrowly/locally about the price increase. The price increase you see is little to do with the operational cost increase of just that game.

Publishers generally tend to increase prices across the board to keep up with operating costs in total. Just because a game is old and doesn't require maintenance and only a modicum of cashflow to "keep the lights on" doesn't mean it's immune to the entirety of profit/loss for the company.

Furthermore, price increases often don't happen on a regular basis in-step with inflation, nor do they necessarily have to follow price increase patterns of other games in their catalogue. Older game often stay the same for years (as others have noted) and then dramatically jump when a budgetary review determines it's finally its "turn" to increase its price.

I use term "price increase" loosely, as the net result is the same regardless if you maintain the base price but lower the fluctuating discounted percentage, or raise the base price and maintain the discount.

And why do they forever increase their profit margin? Publicly traded companies are obliged to forever increase profits to their shareholders/investors. The shareholder buys into the company on the expectation their money will grow as quickly as possible, and so the company does so. Only private companies or partnerships can chose to maintain narrow margins on a whim, because they only answer to a small amount of owners but often they too increase in step with everyone else because they too deserve profit

This isn't a public service. This isn't a charity. This is the economic model our respective countries function on: making profit from selling goods.