Porting website to HTTPS doesn't increase load on the server by much and can be free if you use e.g. Let's Encrypt.
Additionally, HTTPS websites are higher in search results and increases PageRank.
HTTPS is not only about encrypting sended data (like logins or passwords). On HTTP websites malicious scripts can be injected on the client side. ISPs and governments can do it by design and everyone with some knowledge can perform Man In The Middle Attack (MITM) and e.g. add a porn ad on the website or embed a cryptocurrency miner.
Moreover, browsers are deprecating HTTP. Such websites will show more and more warnings. For now Firefox and Chrome show warnings on login pages. Chrome will start displaying all HTTP pages as "Non-secure" starting from July. Firefox will do it too sooner or later.
https://security.googleblog.com/2018/02/a-secure-web-is-here-to-stay.html
https://blog.mozilla.org/security/2015/04/30/deprecating-non-secure-http
You can now access https://winmerge.org