I'm currently
in the sales channel, and
all customers say things like this, then back off the minute a quote is sent through. It's so common it's even easy to spot now:
1. When a manager at some client says "How much will it costs us for you to add $FOO to the product?" I don't even bother updating the sales forecast with the quote I send them.
2. When they say "How soon can we have this?", that's when I actually update the sales forecasts.
So if your sales guys are saying "Look, customer said they'd go with us if only we had $FOO", they're failing the Mom Test[1] - the person they spoke just didn't want to be too negative, didn't know how to say "No" to a charming and likeable person[2], etc.
Sales is a function of the demand in the market. When the demand is (for example) 200 units/m of something, doubling your output does not let you sell 400 units/m.
Also, it sounds that your argument is for software products only, which is a tiny part of the economy. I was really talking about companies that sell non-software products/services - their sales is not limited by software development, it's limited by their market reach.
Even if those companies doubled their developer headcount, it'll have pretty much zero impact on revenue.
I mean, look, I can see you're arguing in good faith here, so I'm trying to do the same, but IME productivity simply doesn't have any effect on revenue, all it can do is lower costs.
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[1] This is such a short and valuable read, that I recommend it to everyone I meet who is trying to do sales.
[2] If you're not charming or likeable, then you shouldn't be in sales in the first place.