I don't get why people feel the need for debt interest to be paid off with funds. The way your business, and indeed a lot of businesses in real life, pay for the itnerest is by stacking more ddebt. This means your borrowing money, to pay off the interest for money you borrowed. It owrks in real-life, it works in this game.
If your more interested in gameplay aspects thoguh then I would argue all being forced to pay off interest does is add an unfun mechanic to the game and makes the game have to be redesigned and balanced in terms of prices and stuff. This is a fundamental part of the game, changing it now would add weeks of extra work at least, and where's the beenfit? What's the good part about forcing people to pay debt? In theory they are punished by being easier to kill, debt is a really big deal in the late-game, but why does that punishment exist? Why do you take on debt in this game? Because you have to pay for fuel, power and life-support. Why should you be punished for not making a life-support resource? This game is about the free market, being adaptable, not set in stone. Suddenly having to make life support, power and upgrade materials goes agaisnt teh spirit of the game, which is to make money.Ontop fo that if you force payment of interest then games are going to take much longer to finish. Less money means slower upgrades meaning less money. Meanwhile that interest is not going into the debt pool, so they are extremely hard to kill. If, currently, you made interest have to be paid off with funds then 1 vs 1 games would last 5-10 minutes longer. That is not a good thing in my eyes.
There is only one way to keep the core concept alive but to add in having to pay debt, and that's by changing the values of everything so that a smaller amount of debt hurts your stock price more, and upgrades are cheaper so you can afford them even when in debt. Oh look, now power plays are over-powered so better balance those, oh look now this, oh look now that. Weeks, nay MONTHS of effort would be spent to change this and bring the game back up to the level it is now, all so that you can make players feel worse about the game because interest payments go from an indirect punishment to a direct punishment?
How about instead of changing the system, people learn to better live within it? You can found early, which forces the opponent to found early, meaning you have control of how much debt he's allowed to take on. If you both start with 70K of debt, he can't borrow nearly as much, he has to play more conservatively. Within the game itself, you can realise that he has a lot of debt and so instead of pushing that upgrade to 5 button, invest in a hacker array, then use the hacker array along with the funds you have on not upgrading to gain more funds, which causes him to be threatened by a potential buyout. Now he's forced to buy his own stock rather than continue to push for more money, slowing him down. With the funds you got form the hacker array, you can either upgrade yourself, menaing you've lost a lot of time but so has he (if he's in bad debt trading the equivelent of 200 seconds for you for 120 seconds of his is a good trade because time is on your side) or you can buy up shares of your own stock, and really dig in to forcing a long game.