What i have been hearing from the Microsoft CRM team is that Jscript is still there, plugins, etc. Just a way better layout, form, and business process engine. There will be a new synchronous workflows, so they fire at real time and no asnyc if you don't want. AND JavaScript business logic engine for simple form UX. When X, show field field, when Y calc shipping, etc. CRM 2013 runs only in IE8+ (plus) and higher. So this might not be bad actually. Windows XP, 7, 8, 8.1 overall these changes sound like they could only help SMEs and Contractors. It does sound like the migration will be a heavy lift but this is expected.
Thomas Day