The phone industry evolves much faster than PC and people cry when their old phones are left out of the new updates, well.. let`s see.. first X86 32 CPU was launched in 1985 since then 31 years have passed with multiple variations of software. Now we have the modern smartphone that started with the iPhone that was launched in 2007, 9 years have passed and we are already at almost entry PC desktop performance levels, with quad core CPUs that can rival Core i3 (Snapdragon 820 does this if i`m not wrong) and 2-4 GB of RAM, the GPUs can also render games at 1080p, i remember my 2006 AMD X2 5000 with Nvidia 9600GT struggling to play at 1600x1080.
The hardware on older phones simply cannot keep up with modern software, and they will not keep the software behind just for the love of some cheap old phone. The industry is evolving fast in this market and sadly some phones will be left out.
PS: in my previous comment i`ve said there will be 2 waves of update, Gabe Aul said that there won`t be any and that some phones have older CPUS that simply can`t handle the new code, guess he`s talking about something similar to instructions sets found on modern CPUs, like when you couldn`t play a game if you didn`t have MMX instruction on your CPU.
Also many talk about app gap, the gap is getting smaller and smaller by the day, only few important social apps are missing but some are already in development while others have very good 3rd party alternatives, some of which are even Universal apps (UWA)
UWA is amazing, i can have the same app and same functionality on PC / Tablet (2 in 1) and phone, they are great and i now understand why MS is trying to force people to switch to Win 10, more users > better market, better market > more devs will be interested to come and develop apps.
The store, even though MS will get a cut (Google/Apple does the same), will be good for them since it`s harder to pirate stuff like this.