When did I miss this?

I bought a Z77 motherboard a couple of years ago with the idea of slapping in a cheap Pentium (which I did at the time) and then upgrading to Ivy Bridge i3/i5 when prices came down. Does this not happen anymore? Am I getting old?

Just checked Scan and it seems that there's only 1 i3 I can buy and 1 expensive i5 and everything else has disappeared...

They're heading towards using their mobile chips as the basis for everything apparently. This has led to 'Broadwell' (late, soon to be superseded already as a result it's said), with a small footprint in every sense but not much of a reduction in processing power. I imagine they've cracked it enough to allow for desktop versions

The speed with which Intel rattles through processor generations is working against you there I'm affraid as they jumped to LGA1150 for the Z87 and 97 chipsets.
I've got a Z97 motherboard and it'll support the newer Broadwell chips whenever they come out with desktop varriants but the ones after Broadwell (Skylake apparently) will switch to a LGA1151 platform.
Looks like, if you jump in at the start of a new socket you'll get one subsequent generation out of it then you're stuck and given Intel seem to manage a new generation each year or so you'll get 2 or maybe 3 years compatibility out of a motherboard at most. On the other hand, performance isn't really increasing that much on the desktop at the moment unless you're doing it as a measure of performance per watt.

One reason why I'm happy to buy laptops these days. You don't have to worry about this sort of thing.
Not that I did with desktops either. I usually kept my CPU so long a new socket type was required when upgrading.
I think I only upgraded a CPU without changing the motherboard once in all the years I built my own PC's.
It was something like changing an Athlon 1400mhz to an Athlon XP 2400+ (I forget exactly what).

Every time I build a PC, I assume that I'm going to keep it for ten years, and as such buy the newest motherboard/cpu combination I can, and then I don't worry about it. Memory can be upgraded, as can hard drives and such like, but I have never and would never upgrade a motherboard or processor.

Being able to upgrade motherboards and CPUs was always part of the fun for me when I was younger. I guess now it's like every other once you're over 30 - boring.

Buying expensive stuff and then keeping it for ages, makes perfect sense as a grown up. I've applied that strategy to my SLR, phone, ski gear and outdoor clothing. But the price of a decent i7 and top motherboard is always eye-watering and I guess I just don't really use it enough (unlike the other things I've listed) to warrant the spend. Ultimately though JV, you're right. Maybe I should just get a second hand i5 and leave it at that. Practically speaking, a laptop would be far more practical these days, with the amount of travel I normally do. I just like having a "real" computer with a big screen, desk and keyboard.

I guess I really am getting older!

This applies to me as well.
My current rig (cpu, mobo, ram) is 6 years old and still more than fast enough for my needs. Only thing I could gain buy an upgrade is Sata 3 ports instead of my Sata 2 (bottleneck with SSDs) My Antec PSU is even older (8y) and still running well.

The only reason I am planning to buy Skylake setup is to get free Windows 10 upgrade that will be limited to one year (apparently).

Regarding your CPU upgrade, I've made that mistake with getting cheaper processor (C2D) and planned to get a better cpu (Quad core) later. But when it came to it I was better off getting a new cpu+mobo+ram combination, and that's what I did.

I now have a Surface Pro 3 tablet, but it has a dock and I use a proper mouse, a keyboard with Cherry MX Red keys and 2 24" monitors when I am at my desk...