Thank you very much for writing to us.
Answer might run long; but I will try to put forward my personal view point
I will start with mobile revolution and rule google came out with in the year April-2015 https://support.google.com/adsense/answer/6196932?hl=en that we might not rank your site to users using google on their mobile device if your site is not responsive.
So after this announcement and basic research of few of our customers google analytics report we noticed people visiting sites using their smart phone is increasing with small to all most double margin then what they visited last time; so for example first 6 months 5% visitor visited using mobile and next 6 months 10% of total visitors; but when traffic of site is not huge it was not accounting to significant loss. But being google accounts for 85% traffic it was an alert; and this who had no responsive site we recommended to slowly and gradually converted to Responsive and off course there was an additional cost to it.
For new clients we started giving them no choice but site with responsive design. Business and SMEs don’t understand technology and so as cost they have to bear for change in technology.
I am sure you are aware that; to make responsive we use CSS and modify it for Desktop, Tablets and Phones; this is better and effective way specially when your site is small and static. The advantage is any changes in content gets reflected on all devices automatically.
Now many time many client ask; not to use responsive and give user mobile experience by creating separate design for Mobile; which I personally don’t recommend unless & until you have huge technical bandwidth to carry on regular upgrades or you have spent huge to make it dynamic though single content management distributing content for two different design layouts.
AMP is a way to build web pages for static content that render fast. It as AMP HTML, AMP JS & Google Catch as written on their open source project website; so if you ask me working on static small site to make it faster; I might not recommend and go with responsive. https://www.ampproject.org/docs/get_started/about-amp.html
But as AMP project getting momentum there are ready made plugin for Wordpress; which makes your post automatically AMP ready; so why not ?
so conclude yes its is now must to go for responsive; but if there is an additional cost no further investment require just for running behind mobile fanciness!