As with other necessary business functions like utilities, the end user pays for services provided offsite, such as remote monitoring and management, help desk solutions, backup and disaster recovery, and more. Managed IT services thus become essential operating expenses to maintain core functionality, rather than additional expenses applied during exceptional issue resolutions with break/fix models. MSPs enable their end users to run their businesses more smoothly and more efficiently than they would otherwise. Additionally, they offer SaaS-based solutions at a price that can’t be achieved with in-house options.
In terms of MSSPs future, I think that we're only now seeing the tip of the iceberg. The reality is that cybersecurity is becoming increasingly more complex, expensive, and essential. I've covered a number of these angles in a recent article https://silentbreach.com/blog_article.php?post=33 . The upshot is that, as you've correctly pointed out, MSSPs are just much more efficient than a typical in-house IT team.
Cloud computing has allowed managed IT services to expand beyond the regions and borders that would constrain the average break/fix IT through the adoption of Software as a Service (SaaS) technologies, as well as Infrastructure as a Service (IaaS) and Platform as a Service(PaaS) also. These capabilities allow managed IT services to scale at a rate dramatically larger and faster than in-house IT operation or break/fix providers.