Every MUA I've used allows the reader to set a font size, so changing font sizes is 100% a feature of plain-text emails. Then they get the link the size they need to read it correctly and it's absolutely easy to read. This here comment is pain text. Is it hard to read this link:

http://microsoft.com/

I don't think so. I certainly didn't have to resort to HTML to make that link readable and clickable.

I don’t have problems seeing and clicking normal text, thank you very much. I don’t want buttons on my emails.

I think the OP app is meant for creating transactional emails (or bulk-send emails like newsletters).

Those templates should account for all types of people and accessibility levels (including things like ADHD, where you need a big red button to click, otherwise you get overwhelmed by a block of text).

You can just send a link, and the user's client will probably highlight it even if it is plain text.

Yea, but how will they hide all the tracking URLs and base64 encoded PII from you in the email?

Using a URL shortener obviously. But you are right, if they only send plain text, they won't be able to include those 1x1 images at the bottom to track whether you have opened the email. Any sane email client blocks images by default, but whatever.