Both have something to gain. Mazda is desperate to develop hybrid and electric drives and was already partnered with Toyota on some of that. ICE is a dead technology walking now that electrics can outperform in every domain except range and price. And that's getting solved much faster than anyone expected. I don't really see HCCI as anything but a transitional approach for anything but specialty cars. But Mazda and Toyota are a very synergistic mix for the transition to a fully electric future. VW/Porsche/Audi and BMW and Volvo are jumping fast into electric tech and Japanese brands have rested on an early Toyota lead on hybrids. Even Kia/Hyundai are out ahead. Hell, even gm is ahead.

I am a long time Mazda driver who plans to buy a Tesla Model 3 if they prove reliable and popular over the next couple of years and if I can make my 2014 Mazda last that long and if electric infrastructure gets built out enough that I can go on road trips with it. A $50,000 version of that car will go 0-60 in 3 seconds, max out at 135mph, and run 300 miles on an hour's supercharge. If they really get that tech down to $30k (I know the base Tesla 3 is $35k, but an enthusiast really wants the higher powered version) by 2020, I don't see how cars like the Mazda3 survive. You'd save enough in gas to make a $30K Tesla 3 cheaper than a $25k ICE compact over a few years. And it will win any performance test to boot.

You just have to drive a Tesla S once on an on ramp to realize ICE is doomed.