> Businesses and investors love SaaS because the economics
> of SaaS are impossibly attractive relative to selling
> software licenses.
I would be careful and not over-generalize. The statement above is opposite of what many late stage investors think of SaaS companies. Their gross margins tend to be horrible. Some are basically in the business of being an intermediary mailbox for transferring money from their customers to AWS.Sure, if you can charge, say $1 for processing one Kb of data, you'll love being in the business of selling SaaS subscriptions. But it's not always the case: for many SaaS companies selling more and more subscriptions means constantly re-writing (AKA "scaling") their codebase, maintaining a large and expensive ops team and dealing with ever-growing AWS bill. Some SaaS companies only have ~10% of their R&D allocated to evolving the product, the rest are struggling keeping it running and it's not cheap.
Compare that to selling a recurring subscription to a downloadable license file. Stripe is a great fit for SaaS, but TWLO will have to face a much tougher reality than MSFT had.
SaaS can be awesome, or can be hell, take it from a co-founder of Mailgun.com
Stock market seems to agree: the 1st wave of software explosion produced lots of highly valued tech giants: MSFT, ORCL, etc. The current crop of public SaaS companies pale in comparison, the new giants don't sell SaaS, they sell ads.
P.S. The billing model is very loosely related to how a software is delivered. Look at Adobe who're happily selling monthly billing plans for good old photoshop.