Let's summarise the history of AMD's socket compatibility over that period.
Socket A supported K7 CPUs over a long period, from Deneb and Thunderbird up to Barton. It was replaced mainly due to the advent of on-die memory controllers in K8. It was straightforward to use a Barton CPU in even an early Socket A board (if you could find one whose capacitors hadn't blown up), but it would run at a lower FSB speed, and thus a lower core speed unless you had an unlocked multiplier.
Over that time, Intel used three totally incompatible sockets for successive versions of the Pentium 4: 423, 478 and 775. Unusually, the latter was also used for the P4's immediate successor, the Core 2, probably because the latter retained the FSB-based system architecture while not increasing power requirements.
Sockets 754, 939 and AM2 were mutually incompatible, but the same CPU dies were made available in 939 and AM2 formats, giving a choice between DDR1 and DDR2 at almost equivalent performance. Likewise the same CPU was often available in both 754 and 939 formats, giving a choice between single and dual channel DDR1. I still have a Socket 939 board fitted with an Opteron 185, which I got to make use of my existing investment in DDR400 RAM (hence I skipped DDR2 entirely on the desktop).
The AM2+ socket accepted AM2 and AM3 CPUs. The latter gave a choice between DDR2 and DDR3, or an upgrade to Phenom II from the Hammer (K8) architecture in the same socket.
Many of the better AM3 m/boards also support AM3+ CPUs with a BIOS upgrade; the main distinction was an upgrade to the power spec. This allowed use of Bulldozer/Piledriver in the same socket as Phenom II. (In many circumstances, it still makes sense to put a Phenom II in an AM3+ board instead of a Piledriver, but that's beside the point.)
AM4 will not be backwards-compatible with AM3+ or FM2+, but that's because it's a DDR4 socket, while almost none of the older CPU and APU dies support DDR4. (The sole exception to date is the Athlon 845.) It will be forward-compatible with future DDR4-based CPUs and APUs, and (unlike AM3+ and FM2+) will also be cross-compatible between CPUs and APUs.
I believe the FM2+ socket was released in advance of actual FM2+ APUs, so many people are able to upgrade from Trinity to Kaveri, Godavari and now Carrizo. This makes particular sense for the dual-core A6 version of Trinity.
That leaves AM1 and FM1 as the oddballs, supporting only one family of APUs each. For FM1, that isn't really a handicap, since what it supports is Llano, whose four K10 cores are frankly the equal of anything in an FM2+ socket; the main upgrade path is by adding a discrete GPU.
I'll grant you AM1. However, it seems this was specifically designed for minimum-cost support of low-end APUs based around the "cat" cores. These cost considerations probably made it impractical to support faster units in the same socket, mainly due to greatly increased power requirements and the need for dual-channel RAM. In short, it was never intended to be socket-upgradable, merely to simplify system assembly and stock-keeping.
With recent Intel families, it seems that they have changed the socket incompatibly without a technical justification. There is no increased power requirement for Sandy Bridge, Haswell, Broadwell or Skylake over their predecessors - quite the opposite, in fact. I could expect a refinement in power delivery quality perhaps once over that period. There is also no difference in RAM support over most of that period, with DDR4 being only a recent change. The difference between dual-channel and triple/quad-channel versions is justifiable, but using different sockets for each of Haswell, Broadwell and Skylake is not.