Just to be clear... Do you want some chat feature built into the Lightning Network software to enable LN Hubs to talk to each other or do you want

a chat application like Whatsapp or WeChat that incorporates the Lightning Network as a internal currency with some kind of plugin? There is need

for both of these applications, but the one for a large Social media platform would be better for Bitcoin adoption.    I think these companies would

create their own Crypto currency, because they would want to control all aspects of it.    You cannot add a currency feature and then rely on

external influences to determine the future of that technology.

There's really no neew building a chat app on top of the lightning network. We already have efficient applications even in the regular network, we need the lightning network for faster transactions such as transacting cryptocurrency. If we want to build a chat application, then use basic platforms.

They will lose in fees. To open channel and to leave channel.
Also,  there are fees in lightning network. But they are small
So in average people will pay to chat .

There are many other "open source private messengers" (even p2p), that don't happen to require satoshis for sending messages. This idea is nonsense, it can be done from a theoretical point of view but its pointless and I wouldn't use it.

Also, once the channel is closed, messages would be lost, as far as I understand the lightning network. The messages wouldn't be on chain.

If you are thinking only about privacy, certainly there are better alternatives for a chat. Look for some Privacy friendly open source chat app here
Privacytools.io

send one word and my friend receive afer one block?

Why not apply end to end encryption or PGP?
LN is not made for that purpose. Even if you want to do it, you'll need to open a channel here and there. Pretty sure end users wouldn't want to do that and just stick with whatsapp with a fake number.

Lightning network has fees when you send something, pretty much like a transaction data, only messages this time. Another thing is that there are already a lot of applications dedicated solely for this purpose.

Another thing to point out is the decentralization that comes into play in Lightning Network. It would bring 2 main disadvantages; [1] It would make private data somehow insecure, since it is stored in individual nodes rather that governed under a centralized secured server, and [2] Even though Lightning Network is faster than the typical blockchain in terms of response time, centralized servers are still much faster generally in performance, which is essential in having an app with the purpose of communication.

For what purpose? I don't think it's needed if you are thinking it's needed for discussion & learning or even getting help. Because, you can learn everything here in the forum. A lot of legendary are here to help you and figure out if any problem you face to. I have recently had some discussion on Lightning Network and it's my pleasure that I have learnt a lot from the forum.
By the way, a beginners guide to learn basic about Lightning Network if anyone is interested- https://bitcointalk.org/index.php?topic=5202798.0

well if you didnt know you wanted to pay G 10 until today you need to set up a channel, which involves an onchain transaction so you might aswell just send him the 10
LN's utility model is more complex than any payment system i know. it involves preplanning spending habits for even a chance of saving on fee's