It's true that AWS has Redshift Spectrum (and Athena) to help with more scalable querying across S3, however I don't think that makes a big risk for another company that provide a focused offering on top. Snowflake is very well capitalized with close to $500M in investment and plenty of customers so I wouldn't worry about them going out of business.
I especially like them because they have the elastic computing style of BigQuery but charge for computing time rather than data scanned, which is a much more effective billing model than anything else out there.
Anyway, at this point I'm just repeating myself so I suggest you actually try them if you care for a better model. It works for us at 600B rows of data that was too expensive with BigQuery and too slow and complicated with Redshift/Spectrum.
We all agree that Redshift is not scalable at some point and people switch to other technologies when they need to but that's not the point. Creating a Redshift cluster is dead-easy and getting the data in it is also not complex compared to other solutions if you're already an AWS user.
The case for Snowflake is different, they also use AWS but they manage your services. Although they did a smart move and store the data on their customers' S3 buckets in order to make them feel like they "own" the data but it's possible only because AWS lets them to that way.
I believe that AWS doesn't try to make Redshift cost-efficient for large volumes of data because they already have the advantage of vendor-lock and making money from their large enterprise customers processing billions of data points in Redshift. That's why there are many Redshift monitoring startups out there to save the cost for you.
On the other hand, AWS is smart enough to build a Snowflake-like solution when their Redshift users start to switch BigQuery and Snowflake and the companies such as Snowflake needs to be prepared when the day comes. Cloud is killing everyone including us.