I usually make the ethical case for restraint. Here I want to make the economic one, because I suspect it's the more persuasive of the two.
I talk clients out of things fairly often, and sometimes out of quite a lot of money. Someone arrives wanting a full rebuild and leaves with a plan for three pages and a note to revisit in a year. That's real revenue, turned down on purpose.
On paper it looks like a poor way to run a business. It hasn't been.
When you sell someone everything they asked for, you get one project. When you tell someone the truth against your own interest, the relationship changes. It stops being a transaction. They stop wondering whether you're selling them something and start asking what they should actually do, which is a different question.
And they come back. For years, for most things, and they tend to send people along, because there isn't much more convincing than an owner telling a friend that you talked them out of half of it.
Sell the whole bundle and you win the first job and lose the decade. Turn some of it down and you tend to keep the client for as long as they're in business. I don't think that's generosity. It's just the arithmetic, and it happens to let me sleep.
Restraint isn't leaving money on the table. It's declining money from a table I'd like to still be sitting at in twenty years.