Did a bit of research and what it seems is that continual light was essentially a permanent daylight spell. Since continual light is already missing from 3e... where daylight and continual flame spell appear, it seems that when 3e was made this spell was split into two different spells.
One of the many reasons 3E sucked...
Daylight spell was more potent in 2E - but was called Sunray. I think you failed your research roll.
Also why do you need a separate spell in PHB for worldbuilding reasons? Don't get me wrong i am a fan of "everyday spells" but there is a limit how much of them are actually needed in rulebooks. Also not everything needs to be a spell that every wizard can use. Maybe the spellcasters of Underdark have learned how to make such powerful light spells, but there is absolutely no need for them to be available to every spellcaster.
I'n one of those types who wants the 4th pillar - civilizing the wilderness - to be in the core again. Fortress construction was part of all editions prior to 4th; continual light was part of this. Players often want to build their fortresses using magic - stoneform, transmute, etc. A couple pages more, or even making certain spells groups of spells in a single entry, would have made room for the two big ones: Rock to mud, and Continual Light.
In all seriousness, Continual Light, the various ___-Form spells, and Transmute Rock to Mud are all spells that give reason and sensibility to depictions of the underdark (and for clothform, woodform, etc, reasons why the clothing is shown as renaissance, while the tech is medieval).
Making the continual daylight spheres with the magic item rules makes them a lot more rare. Too rare to be numeroous enough for agriculture sufficient to live off of. A continual light cast upon a pebble was the default "Hello, Fight!" response from many a party - the wizard would make a few dozen during downtime days.
If you want to have "continual dayflame" spell in your game, then its fine. Convert or homebrew it.