You know I spent a lot of my professional work life on the receiving end of these messages and if I had even one ounce of power to change anything for the better I would have.
In the beginning I would still compile user complaints into write ups for my managers à la “hey these 50 people hate that we do X, maybe we could do Y and win back their gratitude/trust” - but I soon realised that’s just a waste of my lifetime, because PM don’t give a fuck.
And why should they? Even if you improve the thing it won’t matter - the majority of people just want to vent like you; they don’t care anymore if the product improves, even if you would give them the perfect product of their dreams it wouldn’t change their minds.
This might sound jaded but there is a reason why the market is dominated by god-awful products - those that gave too much of a shit were sorted out early enough and only those that focus ruthlessly on the money and only the money survive.
There are a few exceptions of course but those just prove the rule to me.
The flipside of companies not caring is that sometimes they tend to throw you a bone in the form of something they can control, like a free month of service, or a coupon for a free/discounted widget.
Even if the faulty product/service never changes, writing those letters can result in savings of hundreds of dollars a year if done right.