Some bad purchases have nothing to do with money. When AMD/Nvidia re-name their cards, 8800GT is now the 9800GT or AMD 480 is now the AMD 580, and ask you pay a premium price over the predecessor that is a bad purchase. When Intel creates a paper launch forcing people to pay $100 over MSRP even then they are lucky enough to even find the CPU, that is a bad purchase. When Intel launches the budget i5-8400 but has no budget mobos to go with it, that is a bad purchase. When AMD launches a CPU like the bulldozer that shows no real world performance increase core vs core on the preceding Phenom II, that is a bad purchase; at least in my humble opinion.
The 8700k is clearly the better CPU then a 7700k yet if you were building a gaming PC in February 2017, the Intel 7700k was the way to go all the way up until Intel's 8th generation paper launch. AMD has nothing that can touch an i7-7700k in gaming especially when overclocked and the 7700k has plenty of horse power to continue to max out games for another 2-3 years (or longer) since the new consoles are re-using processors from 2013 and we know AAA titles are all built for console first and PC second.
The other issue is price & availability with the 8700k (and other items), I checked last week on Newegg and the 8700k was out of stock (until late November) and going for $450 ($100 over MSRP).
Below is a link to techreports first build with a 7700k from Feb-2017 (I could not find any previous builds on techspot other then Oct 2017)
http://techreport.com/review/31389/the-tech-report-system-guide-february-2017-edition/3
Cost for the 7700k - $350
Cost for a mobo - $125-150
Cost for 16GB 2400 RAM - $100
Cost for a GTX 1070 - $395
Now lets do a cost analysis for the 8700k build today
Cost for the 8700k - $420 ($70 over MSRP and out of stock until the 18th so you may not even get one)
Cost for a similar mobo - $125-150 (a wash)
Cost for the exact same RAM - $186
Cost for the exact same GTX 1070 - $450
So you would be paying a premium of $211 dollars right now to build that 8700K over your February 7700k build for basically the exact same gaming performance (especially once you OC the CPU and are GPU limited). Yes you get a much better CPU for content creation (no question there) but that $211 can go towards your future Intel i5-9500k, i5-15000k, i5-1600k, etc., and the IPC gain in there could blow away anything the current Kaby/Coffee lakes offer (similar to the jump from Nehalem to Sandy bridge IPC)