Careful Spell and Evard's Black Tentacles?

[MaxWilson]

*snip restated argument about wording*

Just think about the effect a 'persistent' careful would have. It completely changes the nature of spells like web, stinking cloud, sickening radiance, and more. Your allies can now hang out in the area of effect, completely protected, and enemies have to make repeated saves just to attack them. This all for one sorcerer point? Careful is now probably the most powerful metamagic for it's cost.

Only against melee monsters with short reach. Don't get me wrong, in the right scenario it's delicious, but it's not universally applicable either. E.g. you can't hide the party inside a Careful Web if monsters with 10' reach can surround it from any angle. They can reach any spot in the Web without entering the Web. So you need to pick the right placement for the Careful Web, and then avoid getting anyone grappled and dragged out. It's a great metamagic but not as simple to use as you're saying.

I think "most powerful metamagic for its cost" is debatable (Quickened is excellent for gishes, Subtle is powerful against counterplay and in certain kinds of noncombat situations, and Extended can double the healing output of Aura of Vitality and lets you long rest without losing Death Ward and Aid--my money is on Extended), but even so, that doesn't mean you need to nerf it into virtual uselessness. There's nothing wrong with SOMETHING being the most powerful.