The only thing that's declining is sales of pre-built PCs and laptops, a lot of the market research is done on Dell, Lenovo, HP, and other big name sales, here's an example. The explication for this is that tablets and other mobile devices are eating into the traditional PC pre-built market, someone who's only going to browse the internet will just buy a tablet instead of a 400$ Dell to fit their needs nowadays.

When it comes to digital game sales and PC gaming hardware though, the market is growing, and growing quite fast. You can find a lot of data on video game sales on pwc's site and PC gaming hardware market report from JPR.

Here's even more research from JPR showing the high growth of the computer graphics industry. Interesting to note is that gaming PCs are growing and are expected to grow while consoles are losing market and expected to stagnate.

Now onto this migration you're talking about, it's not happening. If anything Steam is seeing very high growth, the most recent numbers I could find put the total number of Steam accounts at 125 million - Q1 2015 with 8.9 million concurrent users on steam.
The current concurrent peak according to [Steam's own stats] is 12.5 million, that's 40% year on year growth.