Phew...quite a few things mixed up in there
1. There is not lifespan on a motherboard. If installed correctly and connected to a high quality certified PSU, the motherboard should typically outlast all components that are seated on it (work longer than the system will stay relevant)
2. All z170 mobos will support the next wave of GPUs, yes. In fact almost all consumer boards from the last 3-5 years will support those GPUs.
3. CPU compatibility depends solely on the socket. If the motherboard is a 1151 socket then any CPU of that type will fit. If the next generation processors after Skylake use a new socket, then they won't be compatible.
Generally, make a budget and spend all of it or try to build as powerful or "future proof" of a pc as possible. Upgrading only works partially as you often fall out of sync from various components and their relevance.
It's important to get a strong CPU as the base of the build (i5 is the sweetspot for gaming), which can last several years without needing an upgrade.
The GPU replacement cycle is a lot more regular and currently you can get away with skipping 1 generation before replacing it to achieve high-end performance. If you bought a 970 today, you could wait until the second wave of pascal cards before thinking about replacing it. However as these newer cards are destined to be available in q3 this year, you need to make a choice of either investing in current, mature tech or waiting for new and improved stuff.
Buy min 8, ideall 16GB of ram.
Get an SSD as your boot drive and ideall 1TB of storage on an HDD. This is a standard mix these days and allows snappy boot time and loading times in some applications and doesn't cost the earth.
Make sure you buy a high quality PSU. These can last through 2-3 builds before they have to replaced in some cases.
Keep the PC clean, remove dust, make sure components are adequately cooled at all times.
Keep your OC clean and healthy and don't let it fill up with loads of rubbish and shitty apps.
General tips to keep stuff "future proof"