Conspiracy theories aside, I don't know if systemd is a backdoor or not
systemd is free software, the best defense we have against malware. Unless somebody points out the portion of the code implementing a backdoor, it is reasonable to believe there is none.
but tell me, why has devuan succeeded in getting community support when fedora's and ubuntu's systemd-free forks failed?
Most users sticked to their distributions when they switched to systemd. Those users have either seen no difference (almost all "simple" users), except maybe a faster boot, or improvements (mainly server administrators): no reason to switch to a fork..
That anti-systemd public is probably mostly composed of users who, like you, have no technical argument against sysyemd but who decided it was bad anyway. Maybe because they read articles such as http://www.infoworld.com/article/2608798/data-center/systemd--harbinger-of-the-linux-apocalypse.html that predicted "the Linux apocalypse" because "Red Hat has released RHEL 7 with systemd" or, by the same polemicist, https://www.infoworld.com/article/2608870/linux/you-have-your-windows-in-my-linux.html that predicted a "schism over systemd [that] could lead to a separation of desktop and server distros, or Linux server admins moving to FreeBSD".
That was four years ago. Since then, all major GNU/Linux distributions have adopted systemd by default, there has been no "Linux apocalypse", no schism between desktop and server distros, FreeBSD has not suddenly become more popular (on DistroWatch.com, it was ranked 17th in 2014 and 30th in the last 12 months), and Red Hat has almost doubled its revenues: https://www.sec.gov/Archives/edgar/data/1087423/000108742318000010/rht-10kq4fy18.htm#sF656807914B25FDB96A1C024F2D288CB
The prophecy failed. The well-studied mechanisms to cope with the cognitive dissonance are at work in the (small) anti-systemd community: https://en.wikipedia.org/wiki/When_Prophecy_Fails
