You should always look to design your product/service to be scalable. This isn't limited to Bitcoin, but to everything you are looking to code. However, its especially true for Bitcoin, if you want to accommodate as many users as possible, then implementing features which limit its expansion isn't a good idea. If you want an example outside of Bitcoin; if you were designing a website to login users from your local community, but later you decide to expand nationally, but didn't implement a way to easily scale your website, then that might mean you have to redo most of the code in the future. You want to avoid that. Bitcoin, because a lot of changes would require a hard fork, and that hard fork then has to reach consensus it quickly becomes problematic.
If you want to learn specifically the problems with Bitcoin, and scaling it then check out this article:
https://en.wikipedia.org/wiki/Bitcoin_scalability_problem