On a personal note, speaking as a former M1 enthusiast and someone who has agonized over hosting, security patches and development pipelines for more than 10 years, it’s refreshing to relinquish the brain space of these issues and focus on customer experience. This is possible by moving onto SaaS-based ecommerce platforms and having a performance SLA in place.

- Keith Karlick Principal - Mercutio
Along with greater headspace, there is a tangible savings to be had with SaaS ecommerce. Many of our clients see savings in total cost of ownership (TCO) over traditionally hosted ecommerce. This is due to productized integrations and leveraging a vendor ecosystem to bring most of the functionality to the site, leaving the bespoke work focused on the customer experience and custom features.
For any business considering staying on M1 past the end of life, security will become a major issue; Adobe is ending security patches and ongoing updates for M1. Most sites running M1 rely on Adobe to provide security updates, and as new vulnerabilities were discovered, Adobe released patches to fix these issues. With Adobe no longer providing that service, it will be up to the merchant to test and repair patches within the Magento application.
Magento is built on an open-source hosting technology which also needs to be kept up to date. The last supported version of PHP is 7.2 for M1. As PHP receives updates, Magento may or may not run on the latest version of PHP. So not only will the application be out of date but the server software will be as well.
Once the decision is made to migrate off M1, there are a number of considerations to think about while planning the project. BigCommerce, Shopify and Magento 2 are the three largest players for the mid-market of ecommerce.