The truth is that the world is a complex place and you don't always know whether you can keep your existing customers by improving what you already have or get new customers to decrease the risk of declining income. It is hard to measure what is acceptable tech debt and what is worth addressing.
In one sense, the proof is in the income. If you can help customers with an 80% OK product then you just need to live with the dodgy 20% bit.
I agree with the danger of big customers though! If you ever think something is a way to avoid hard decisions, pivots and annoying some of your customers, then it is too good to be true ;-)