and we can see it already takes people a considerable amount of time to download the entire Bitcoin blockchain with the purpose of running a full-node.
That's true but remember that they have to do it
only once. You don't download the entire blockchain every time you run your full node!
Think of it as installing a new operating system. It takes a lot of time to format your disks, re-partition them, install the OS, update it, install all your software and maybe update those too. If it is not your first time and your internet speed, your hard disk, etc. are slow then it could easily take a full day to do all these. BUT you only do it once, every future time you start your computer the operating system starts from where you left off.
Initial Block Download is the same. You do it once then each time you restart your node it starts from where you left off and the amount of data your node has to process is much less.
