You no longer need to run fullscreen to take advantage of SLI, at least not on Nvidia cards. The difference between window/fullscreen is just a few FPS - Since I still run my triple monitor setup, I need the extra card if I want to run "Ultra" and keep a steady 60 fps.
Since they switched to AFR mode for like, all games nowadays, SLI and Crossfire works pretty much across the board and gives you ~90% increase. Downside is of course massive heat and poor heat removal from between the cards, especially if you don't run reference design.
There is a huge downside though, because it's very difficult to upgrade - If I want to upgrade now, I need more than a 2080ti, since that is equivalent to what I run now. Same with the surround monitor thing, if I want to upgrade these aging Dell IPS 1080 monitors, I have to buy 3 new ones and well, considering the amount of games I play nowadays, it's very hard to justify.
That being said, unless I increase the amount of pixels by buying new monitors with higher resolutions, these GPUs will last me quite a while longer, as long as they don't break.