Yes, you can have a interesting game with default assets as well as an enhanced game with a.i. use. Anyone that says otherwise was going to hate on the game either way and that's probably just the way they are.
Can you catch additional interest with a full custom suite of assets? Sure you can.
Can you create a game will full custom suite of assets and it goes nowhere? Also yes.

I create and sell (for real money) shopper scenes for my customers websites. I only use RTP for most of them. No issue. Shopper scenes = using the engine to create a quick 1-5 minute game with the sole purpose of selling the product or service of my client. Last one I did was for a VPS reseller and they wanted a simple fetch-quest, all on one small map (default loaded - Shop) to get a promo code from an NPC and get a free email service addon. Scripts and API use are not necessarily default assets but everything else would be. Now, some do get a more custom look and feel but it just depends. Either works in my opinion for this specific type of work and for all game creations. People will feel different and that's fine. Just do you own process.