I know this can easily be done but the centralized website would be for normies to use with clean UX/UI. There could be hundreds of versions that fit each individuals wants. Think of a personalized twitter feed.
Twitter, Instagram, ... all have dedicated clients, usually only for phone but sometimes desktop version also. If you are going to design a decentralized message platform it is best to keep everything decentralized.
Why do you think the tokens (if structured the right way) couldn't drive incentive? Each token is worth one message. This would have to be well thought out but it would eventually become a "pay-to-tweet" service which is fair because there has to be a small cost for the service. Just as there is a small cost to use any blockchain technology.
Because you can't have everything. If people are supposed to use the coin to pay for their tweets then the price of that coin has to be low. You can't keep the price low, the market decides that. If it goes up, people won't be willing to post and if the price dumps the chain becomes insecure. The price has to be enough to create incentive for miners to mine that coin and if the difficulty goes up it has to go up to justify the increased amount of work they have to do.
Isn't this a risk of any PoW chain?
Yes but doubly so for a new project that has a good chance of not gaining a high value.
Experience suggests that people aren't going to abandon what they are already using
for free for a new platform that they have to pay for.
Low demand means low price and worst of all it means smaller market that is very susceptible to pump and dumps.
P.S. The only viable solution is to build such a project on top of an existing secure immutable decentralized blockchain. Like a bitcoin side-chain that is merge mined by same bitcoin miners.
There may be other solutions like using a different algorithm other than PoW that I can not think of right now.