It's a fascinating ecosystem, but a little confusing. I was hoping there was an easy(ish) way to develop a custom theme, and use wp.com for hosting -- but as far as I've been able to figure out, there's no way to do that -- to use wp.com is to opt out of any serious customization. On the flip side, you get (presumably) rock solid wp hosting, and few security issues.
Every time I've considered wp, I've ended up turning away - the code base is quite awful for the basic features it provides, and as soon as you start adding third party plugins (for added features), you need to be very careful to avoid either breaking on updates, or security issues (with a plugin, or cross section of plugins). It feels like half of the traffic to full-disclosure is related to wp plugins.
I'm not the typical wp customer of course, but for me the ability to develop some fairly static (html+js+css) themes along with wp hosting might have been interesting. If I can't use custom themes, wp.com hosting isn't all that interesting -- and if I have to host it myself, I want something that's a more solid software design than wp (and hopefully maxes out at around a single db query per page view. Something which is easier if you can assume the db has proper views, for example (select * from anonymous_page_view where page_id=?)).
I see that the same is true for the elegantthemes (which appear to be terrific value at ~250 USD for a life-time, unlimited license, btw):
https://www.elegantthemes.com/join.php#wp-com
"Can I use your themes with WP.com?"
"Unfortunately WordPress.com does not allow the use of custom themes. If you would like to use a custom theme of any kind, you will need to purchase your own hosting account and install the free software from WordPress.org. If you are looking for great WordPress hosting, we recommend giving HostGator a try."
Yeah, I don't think I'll be suggesting clients to use HostGator as a web host.
Wrt. themes and wp.com -- I can see how I could make a few themes and sell them at an affordable price -- I don't really see how the economics of support works out when you need to deal with the entire gamut of outdated php/mysql versions and the plethora of combinations of server+(f)cgi/php-module+mysql/mariadb/otherdbs(?)+windows/solaris/bsd/linux for support and testing. Not to mention interaction with hundreds of buggy third party wp plugins.
> I'm half tempted to abandon my current plans of startup world domination and just make WordPress websites for local businesses.
Best of luck to you :-)