I think it’s fine to include a backlink in the theme, as long as there isn’t any obfuscation of the footer code. I remember back in the day people used to “encrypt” their footer code. Even if you emptied out the file, many times the layout would break or otherwise make the theme unusable. This was done on purpose by sketchy theme authors to discourage users from removing their ever so precious footer links.
From a theme developer’s perspective, I’d consider nofollowing backlinks to your own site in your themes, and/or not using any spammy anchor text like “WordPress themes” but rather your brand name only.
Google’s algorithms don’t take very kindly any more to a flood of footer links on a bunch of random sites, something that can happen very easily after releasing a popular theme. Nowadays, it should be seen as a traffic/marketing/branding tool, not as a way to influence search rankings.
Eh, I don’t like the sound of that. Which themes offer that option? Could be seen as spammy by search engines if you’re
display: none'ing random links in your footer. Luckily a ton of themes are 100% GPL now so you’re free to remove backlinks in the markup as well, manually.
I don’t think it’s really necessary to provide an easy-to-use option to edit the footer link. An “agency” should be skilled enough to make a simple child theme to modify it. Although it would be nice to have, I’d consider it a bonus feature or something to cover in a supplemental tutorial.