I don't see how it does, actually. Any other ratio could have been used, or classes could have been given explicit 'caster levels.'
Technically it could have been, but that was working away from the simplicity goal.
Not surprising, but the 3e bard was, as you point out, also technically a full caster, just one with a slower progression that only reached 6th level spells.
Except 3e didn't rank spells the same way it does now. The same spell would have different spell levels to adjust for when a class was expected to gain it. That caused higher level spells to be classified as lower level spells for bards so they did get 8th level spells for other classes at similar class levels. That was, of course, further complicated by the DC calculation so the same spell would have a lower save DC for the bard. In essence they got the same spell roughly the same time and the mechanics made it easier to save against anyway.
Spell song DC's were still high instead of the spells, however. Fascinate DC was the perform check (which gets ridiculously high) and other spell songs like suggestion or mass suggestion were based on class level instead of spell level for higher base DC's than 9th's level spells at 20th level.
So while spells themselves had a lower DC, the songs had better DC's to replace those of higher level spells. 29 spells of 1st thru 6th level vs 24 spells (30 with specialization) of 1st thru 6th level for a wizard is very similar. The difference was 20 high DC spell songs vs 12 (15 with specialization) higher level spells. Combined it was 49 spells/songs vs 36 (45 specialized) spells on a wizard. The bard increased versatility via skills and the wizard had a broader spell versatility plus metamagic feats to improve spells. That's very similar to 5e where they have the same spell progression but the versatility in bards comes from skills and much less inspiration but the wizard adds a spell recovery option and subclasses improve spellcasting instead of metamagic feats.
Oh, they have been, though you can debate to what degree.
We've had those discussions and I chose to respect your right to your opinion, lol. Agreed to disagree and moved on. I don't see the issues in the games I play so I base my opinion on that.
And, I don't think it's irrelevant to the Bard's WotC-edition arc: from 3rd Tier compared to the Wizard's Tier 1 in 3e & the butt of jokes, to nominally balanced with everything from Wizard down to lowly fighter in 4e & rehabilitated as a 'leader,' to finally being the traditional Wizard's near-equal in 5e & kinda baddass.
It's only badass based on the presumption of the wizard's efficacy, but being comparable is solid anyway.
I find bard power can be lacking in direct comparison. It's more noticeable at higher levels after other spell casters get the goodies to help their spell casting that bards get to improve skills instead. Plenty of versatility, but compared to even just and extra cantrip, arcane recovery, and the difference in ritual casting mechanics it's clear wizards make for better spell casters. Let alone spell mastery, signature spells, and subclass bonuses.
Just because both classes use the same spell progression table doesn't make them equal as spell casters. It's only part of the story.
The Bard's quite a success story.
Totally all me and only me and my awesome playtest feedback, lol.
And a pretty serious alternative to Cleric. (to nod towards the actual topic)
Absolutely agree, with the caveat that spells known by taking the alternate-to-a-cleric route restricts knowing other spells. I find heroism for temp hit points can be much better than using the same spell slot for healing (concentration limitation sucks though) and song of rest can add up to quite a bit, especially in wilderness adventuring.
Clerics get more spells available by a wide margin (and therefore more rituals available for use), better armor, another cantrip, and the caster focused domains apply bonus damage to cantrips bards don't get. That shows clerics are also a bit better than bards as spell casters before adding things like domain enhancements to spells.
Even sorcerers have 2 more cantrips, font of magic / sorc points, and metamagic for more potent spell casting. Warlocks have several enhancements for eldritch blast and several decent spell-like abilities at will via invocations.
The closest bards get to other full spell casters is the base druid. Same spell progression, same number of cantrips. No spell spell enhancements inherent to the class or bonus spells available like cleric domains in the base class. Druids might still edge bards out because their prep mechanic allows swapping spells out more easily, they prep 25 spells vs 22 known at cap, and eventually they all get the beast spells ability.
When it comes to full spell casters, bards are the bottom of the spell-casting list and spell secrets at higher levels tends to get a bit over-rated. What's nice about them is skills and bardic inspiration dice more than the spell casting.
Not all "full" spell casters are created equally, and the comparison of bards because "full" spell casters is actually false equivalence.
Clerics have more cantrips, better armor, many more spells prepped than bards know and therefore more rituals available, and caster oriented clerics gain damage to cantrips bards do not. Cleric domains often offer spell improvements or additional abilities.