CRM won't be supported on that OS given that CRM 2019 only supports Windows 2012 and 2016. For Sage 300 2017 application server, only Windows 2012 and 2016 are supported. I would seriously upgrade. 2017 is well out of support now.

Are you saying the software won't function correctly on Window 2019 or that its simply not supported by Sage?

The client is on a perpetual license for the software and they are not ready to start paying $25,000 per year for subscription.

The odds are will it probably work just fine.  But, there have been major security changes over the last few years, in particular, the deprecation of TLS 1.0.  So even though the product might technically run, it's a security risk.

It may run. What Jay said is true. There are 2 reasons to upgrade besides being supported by Sage on a supported platform: Security updates and legislation changes may often mean your current old system no longer meets current requirements. They may not have much choice. Sooner or later an update will happen on the new environment that will stop the old system working and then you will have no time to prepare and they will have massive downtime and a huge cost to fix the problem. Software has a lifespan and the customer needs to be made aware of that and the reasons why from the very start. You need to document the fact that you told them this so that when it all collapses around them because they don't want to pay out to maintain an up to date system, you can say that you did warn them.