Blue Mage being equally capable as a DPS, tank, or healer seems to be a common misconception. While yes, Blue Mage is known to posses powerful mitigation or healing through spells such as Mighty Guard and White Wind, often times, those two spells were all that Blue Mage had for support utility. Across several games, it's normal to see approximately 80% of their spells as offensive in nature, whether it be through direct damage or negative status effects.

SE knows this. There is a reason why it's been officially classified as a Magical Ranged DPS. And when DPS jobs can have support utility like Mantra and Troubadour, I wouldn't consider it remotely game breaking for Blue Mage to have spells like Mighty Guard and White Wind while being classified as a DPS job. I also wouldn't consider it to be a disservice to the identity of Blue Mage if it can't tank or heal as well as it can deal damage. The only case where Blue Mage was truly that diverse has been in FFXI.

The representation of Blue Mage in FFXI was a deviation from the norm. But in its defense, it could also be called an expansion of what came before. And it's quite clear that FFXIV is borrowing heavily from the FFXI representation. However, when you consider what it means to be a full-fledged tank or healer, and consider that Blue Mage has been designed as a solo job, is it even being created with those capabilities in mind? If Blue Mage were being designed to be the most flexible party job, it would be a different story, but it's designed to clear content on its own.

Being equally capable of dealing damage, tanking, and healing isn't something that is required of Blue Mage in FFXIV. At its core, Blue Mage has a consistent focus on DPS, and while it will possess mitigation and healing spells, those spells may primarily exist for its own benefit when it's being designed as a solo job.