This is for the few who still use desktop/tower PCs, as laptops dominate the show and usually only have room for one drive. SSDs don’t get cheaper as they get larger like HDDs. If you double the capacity of a hard drive, from 1TB to 2TB, they jump from roughly $69.99 to $99.99, give or take a few dollars. If you double the capacity of a SSD, the prices doubles: from about $100-$120 to $220-$250, give or take.
That’s because you can increase drive capacity with a single platter, which is much cheaper than the many chips needed to double SSD capacity. You have to double the number of chips in the drive to double the SSD’s capacity, and that price is fixed.
While it’s nice to put apps on the SSD, in a lot of cases it’s not necessary. Once it loads, the bulk of the drive access is done and it comes down to memory for performance, and most of us have plenty of that. So unless your app does a lot of drive access, such as a compiler, an SSD is pointless.
Also, some lower-end, cheaper SSD drives are known to slow down as they get full, so keeping them at under 50% capacity is actually advisable. It’s easy enough to do that. Every app, when being installed, asks you where to install it. So you can put it somewhere else besides the SSD.
To me, the optimal setup for a desktop PC is a SSD boot drive no bigger than 256GB and a big D: drive, 1TB or more. You will have a fast, responsive PC for the OS and swap file and your apps sit alongside the data on D: drive.