Bitcoin Forum

As the price of bitcoin has risen, the number of users in the entire market has continued to rise, and network congestion often occurs. These transactions between the two parties can occur on the payment channel established by the Lightning Network.

Why use a decentralized Lightning Network and bitcoin needs to be scalable?

There is no such thing as "trusted third party system". Who or what determines what is "trusted"? Something trustworthy to me doesn't have to be trustworthy to you. In Bitcoin, we trust the consensus rules. We trust that there is a law and those who don't follow it will get banned and blacklisted. Nodes don't trust each other, that's why you have thousands of them. Everyone is checking and verifying what the others are doing, and if they discover something malicious, they single them out and prevent further communication.   

Bitcoin cannot have a third party. If you think so, you are wrong. Bitcoin does not support any third party. It is just a peer to peer.

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