Sounds like they don't know the market and are looking for an excuse to not invest.
In general, it's just a question of "is this at least a $1B market". Find a precedent from another industry, that also helps make the big market case like the "we are the 'Microsoft Word' of speech documents".
It should be stick with a unique business strategy and incredible upside. And, It would be helpful when no one has a market perspective as you have yet.
Or you can find people who are experienced in your market to some extent.