This is for who are unable to do actual pushups. These are almost as good as the conventional ones. When it gets too easy, I do one-arm wall pushups. These are really intense and I always feel the burn.
When you are doing core workouts or any cardio, it is better if you do not arch your back. It does feel good to do so, but it is not recommended.
Keeping the back straight imagining that it is stiff helps in both cardio and core work.
The same workout everyday makes you bored and isn't fruitful!
Change your workout , try different videos, dances, martial arts!