So when you think about this for $20 a month. It's a no-brainer in that scenario. We already have on-staff copywriters, marketing devs, social media managers, and so on that could have taken over some of the emails using this.
I might still out-source major custom email designs for huge promotions and stuff. But we could easily handle smaller flash sale and other simple emails internally, using a tool like this. So $20 could be justified even if it saved us from outsourcing one email a month.
For the average blogger, $20 a month could still be worth it if it they have a decent email lists and are converting profit from that list.
If you are just running a personal blog and want to send out fun emails to your list of 100 people, it probably isn't worth it. But most ESPs also have WYSIWYG designers that would be "good enough". If you are somewhat-technical then a tool like MJML would probably offer even more than the above tool offers, while being free.
I don't think Microsoft365 is a fair comparison. One is a mass-market product, the other is a niche indie product. $20/mo isn't for everyone. But at $240 a year, that's less than the cost of out-sourcing a single email every year. So if you can make 1 email a month from it, then you are clearly coming out far ahead.