it's nearly impossible to download almost 500 GB of the entire Blockchain with the speed of the Internet I'm currently using.
what would happen to Bitcoin if these 75k node operators decided to shut down.
You have to keep in mind that in a decentralized and global network if you can't do something then there are still a lot of others that can. For example if you have a problem downloading the whole blockchain because of your internet speed, there are 75k others who don't have that problem and have done it already.
It's the same with other aspects of Bitcoin like mining. You are not willing to make the investment into mining equipment to mine bitcoin but there are lots of others who are.
That's the beauty of being decentralized.
And to keep it that way, we have to ensure the characteristics of the network remain a certain way to keep it possible for enough individuals to continue doing what they were doing (running nodes, mining bitcoin, etc.).
For example if the block size were to increase to 1 GB, it would immediately centralize Bitcoin to a handful of nodes run by malicious companies.
So in reality if we look back at history, we have seen rises and falls in the number of full nodes but it's a trend and won't happen altogether. You can see the chart showing such changes here:
https://luke.dashjr.org/programs/bitcoin/files/charts/historical.html