Why pay a subscription?

[Felis]

I read that they intend to make the current end game content easy when they introduce new end game content.

So why subscribe, the smart man would just pay for the subscription as they update.
So each time they add new end game content they will nerf the previous end game content.

Sounds good to me. .

Only if you want to be always a hiker (casual player)
If you want to be a pioneer (hardcore player), there would be always a new montain you can climb up without hiking paths.

You will be able to do the whole game solo or with the duty finder, no need for guilds,friends,or groups.

Best single player game in disguise

Hmm, no?
Like Yoshida said already, there would be always a new hardcore content where you need a party with hardcore players in it.
In short, have fun to do the Bahamut dungeon turn 6-10 with a df group.