I always wonder if those very-narrow fields - Salesforce, SAP, Microsoft Dynamics, so on - are worth the tradeoff of locking yourself into one environment.

Salesforce and SAP aren't going anywhere, unless they pull a Netflix-style suicide. Depending on your skillset now, they may/may not be worth it. Entry-level Salesforce jobs are tough to come by because the market is saturated with people getting the first admin exam.

SAP guys make good money... but you could do Oracle + SQL instead and really be future-proofed. Everything is data-driven now, so being involved with the data is a non-apocalyptic job guarantee. Hold the data (cloud admin in data warehousing)/ maintain the data (DBAs)/play with the data (data analysts), and you're set. AFAIK, Dynamics is not on the same tier in the ERP/CRM space.

Automation and cloud have allowed the other IT genres to reduce their headcount. You can push a new config to 40 routers in a keystroke now, whereas ten years ago you would have to log into each one and copy flash run. Economies of scale have caused many companies to stop maintaining their own datacenters and server infrastructure, so instead of needing a bunch of sysadmins building and maintaining infrastructure, you only need a few cloud admins maintaining things while Azure/AWS has their internal DC staff swapping drives and performing maintenance.

If you've got a head for automation, there's lots of potential there as well.