We see this question, in Blogger Help Forum: How Do I? , a couple times / year. Can I keep using a Classic template on my blog - or must I upgrade? Right now, Blogger won't require anybody to upgrade. You can stay with a Classic template, if you want. That said, there is no guarantee that the next major Blogger problem will not leave Blogger Engineering deciding to cut their losses, and move forward. If we can get Blogger back online with Layout and better today - as opposed to Classic templates, next week - let's drop Classic templates, and bring it back, today. They've moved forward, this way, in the past - and next week, they could, again. Eventually, this will happen. One day, Blogger will come back online - but without your blog.
What Blogger won't tell you