You're making the wrong assumption here.
They're basing it off how SCH would be able to put out pretty OK damage in addition to healing with minimal extra effort, but they still don't come close to a summoner's raw damage since the summoner has their pet and is spamming ruin instead of healing.
If you replace DPS with more tanks or healers, your total numbers go down and you won't be able to make the new DPS checks. If anything, it will encourage attempts at solo healing and/or solo tanking since primary DPS classes have always and will always outstrip main healers and tanks in terms of DPS.
EVERY fight in FFXIV is timed, so just because you can DEAL damage doesn't mean you can deal ENOUGH damage.