Stop overanalyzing it. This happens countless times every day in every industry.
People start businesses, run them for decades then at some point once they reach a certain age they have to make the decision of whether it makes sense for them to keep running the business and bank the annual profit OR simply sell for the cash. Think about that one uncle/aunt you have that owns a bunch of houses/buildings that are paid off. Yes, it's great to be able to collect rent w/ little work. But they probably stay up at night thinking about how if they leave that shyt to their kids, there's a 50% chance they're going to treat it like a party house, borrow against it then eventually lose it to the bank. So what do they do? Sell that mf and enjoy the money or at the very least they're able to decide how that money gets spent/doled out after they die.
From the buyer's perspective, one thing ppl need to realize is that the easiest money in the world is to have an asset that is appreciating, then basically selling that asset (or at least a piece of it) to large pension funds and other investing groups. You then charge them an annual "management fee." Because they are very conservative investors, all they want is a higher rate of return than if the money is just sitting in a savings account.
These guys will eventually go to the pension funds etc and say "hey guys we own the rights to all this music and every year we make X amount of dollars from streaming/commercials/samples etc etc...we will sell you the rights to the future income for Y amount of dollars + an annual management fee of 5% of the total value of the investments." And if you can guarantee 5% rate of return on investment, you will have a line of billion dollar investors waiting to give you their money.