I've been a rabid supporter of Drupal; Of course anybody who knows me has watched my enthusiasm wear thin for various reasons, including simple burnout mostly, but also a number of frustrations with lack of progress in the community, especially with simple things like solving the support vacuum. However, I do not have an agenda against Drupal in any way, and as a professional programmer, I wouldn't be surprised if the core changes being introduced in Drupal 8 are a step in the right direction for future maintainability and sustainability. (Disclaimer: I have mostly ignored all of the changes and have only a rudimentary understanding of them. I wore myself out on Drupal 7 and swore off Drupal 8.)
Technical debt: But in my personal life I've been using Drupal for almost a decade now. And like everybody else I was sucker enough to say "I know how to do that" when my friends, family or organizations I was a member of had a problem that required a website. Oops. So now I have probably a couple of dozen websites that I am the de facto maintainer for. Already I've been through major version updates for many of them. And for a simple brochure site it's sometimes not too difficult. But for a more complex site with lots of contrib modules, it can be daunting. And then for a site that has custom code it can be overwhelming. The site I've been supporting since 2005, Warmshowers.org, (still on D6) has nearly 40,000 members, serves a vital role, and has a budget of about $3000/year, mostly spent on hosting. So what am I going to do about that when D8 comes along? Even upgrading to D7 will take probably more than a couple hundred hours of work, lots of custom code. D8 is unfathomable.
The forkers of Backdrop have promised to slow down the rate of API change, and to be specifically attractive to small sites without a perpetual budget for rewrites. I need that very much, whether or not I invest in learning Drupal 8, which may be loads of fun. I really need a simpler upgrade path, and one that won't force me to redo everything every couple of years. If I help somebody with a website, I no longer want to have to say "Oh and by the way, you need to budget at least $5000 for an upgrade every couple of years." I really never want to say that again. I need this. Or I have to get help breaking my addiction to helping people with web communication problems.