Well, it is “justified” when it sells and makes gamers happy. And many remakes / remasters sell well and are appreciated. Certainly toppers like Tony Hawk’s Pro Skater 1 + 2 and Demon’s Souls for example. The target audience is a combination of older gamers with nostalgia and newer gamers who thought the old game felt too old or did not know it.
These projects are considerably cheaper to realize than a completely new game; profit is the main motivation for the publishers. Especially with a remaster that mainly increases the resolution and graphics settings is the cash register, but also radical remakes are still cheaper than a completely new game (because no new setting, no story / concept phase, the gameplay feeling can simply be simulated instead found, and often reuse of music, voices, graphic assets, motion capturing, etc.) and represent a much smaller risk because the game had already proven itself.
The asking price is usually a bit lower than a new AAA game, but it is relatively high in relation to the development costs and some remakes go for € 60 over the counter. Not that that is always unreasonable, if it is a good game and a good remaster.
It is a credit to publishers when they give owners of the original game a discount on the remake / remaster, and unfortunately that does not always happen.
Finally, fans of new titles don’t have to be very upset about the many remakes. Usually these are made by separate teams, with relatively few people, and they bring in money that can go to new projects. As long as new titles come out, you don’t really hear me complain.