“How do I know you'll still be around in a few years.”
Why they say it. Betting on a smaller or newer vendor is a personal reputation risk for the buyer if you fold. They are asking you to help them justify the choice internally.
Do not: Do not get defensive or oversell your funding and growth, which reads as bravado and amplifies the concern.
That is a smart thing to be asking, your reputation is on the line with whatever you pick and I would ask the same. Rather than just point at funding, I would rather show you how we protect you regardless, things like data portability and contract terms so you are never trapped. Would it help if I walked you through exactly what happens to your data and access in a worst case scenario?
Then ask. What would you need to see, contract terms, references, or financials, to feel confident betting on us?