There are other ways of bootstrapping businesses: people pitch in money in exchange for owning a percentage of said business. That's exactly using someone else's capital to build, but without the parasitic and immoral practice of lending with usury (aka interest).
That's not bootstrapping, just investment funding. We have that, though many still prefer debt financing instead for whatever reason. For example you can often get money "cheaper" via debt versus giving up too much ownership.
Preferring something doesn't automatically make it good or acceptable. Just like how some people prefer to smoke or prefer to gamble or prefer to drink.
Regarding your point, debt today is only cheaper because it is available and widely pushed by the government through banks. If lending money on interest were hypothetically banned, then everything would have to change, and we'd have a fair equilibrium.
What exactly is the moral framework that makes equity financing okay but debt financing not? The financier is still getting a consideration in exchange for his capital.
