How do you validate the sustainability of an idea?

Validate the idea by testing it in the market as quickly and cheaply as possible. There are a lot of YC content about this, but I usually just reach out to friends based on my first audience assumption, post to reddit & hackernews. Launch as soon as possible and as minimal as possible to get feedback from early adopters, or if not you can do market research to understand the potential demand and competition it can be question from reddit/quora or prolly bad reviews on trustpilot. Focus on validating your assumptions of the idea iterating based on feedback from customers and the market.

This is what I recommend at https://microsaashq.com - The best way in the current trend is to build a landing page, improve wait list user signups by posting to places like BetaList, Product Hunt, closed communities, building super small projects/lists and drive traffic to the main site. Then talk to users 1:1 and see the vibe. Leverage Ads if required. Keep the momentum and get early feedback as much as possible.

The best way to validate an idea is to make a landing page, put it in front of a lot of people and do some pre-sale with discounts. Unless people pay for your product you can never be sure if its worth building.

creating a landing page and seeing conversion seems possible like other people here say, wondering how it can be different for b2b products

Besides what other shared, I often go to reddit and seek peer feedback there. It has been surprisingly very helpful!