No matter how much people keep saying this, it isn't true. The number of machines where coreboot provides a complete replacement for the system firmware is tiny. It'll generally have little to no support for anything beyond basic system functionality. It has almost zero support for TPMs. Support for modern GPUs is marginal, so it's unhelpful for most systems with integrated graphics. In almost every case it would be easier for you to hack your existing firmware than it would be to attempt to get Coreboot running, except you'll hit exactly the same problem you would with Coreboot - systems are unlikely to accept unsigned flash updates, so you'll need a physical SPI programmer to get it onto your system.
But let's imagine a future where system vendors do the additional work to get Coreboot up to scratch for their boards and ship it from the factory. How different would it be to the present day? Well, the good news is that you'd get the source code (hurray!). The bad news is that nothing else would be different. The systems would still be UEFI based. They'd still implement Secure Boot. You still wouldn't be able to reflash them with a modified version. There'd still be bugs that Linux would have to work around. It's a net win, but not a huge one.