What you describe is like the inverse of 90% of the reason companies host in the cloud. What makes needing to forcast and reach out to a sales guy to eventually stock hardware for your needs (while now competing against other customers for those resources) any better than hosting on prem.

AWS for sure has had resource constraints in different AZs (especially during flack Friday and holiday loads) but I have never had an issue finding resources to spin up especially if I was willing to be flexible on vm type.