you can alpha anything with enough fighters. the problem is that any competant UNSC opponent will have his SMACs surrounded by a fleet, and that fleet will have stalwarts and fighter support. this is where the supercruiser comes in, being able to just zap the SMACs from safety.
Firstly, while you might be able to alpha anything with enough strikecraft, there's a difference between intentionally massing hilariously huge numbers of strikecraft and having 4 or 5 capitals with a combined total of 40 or seraphs. That's an easily achievable, practical number which doesn't require you to gimp your fleet by just spamming SDVs or something. Even if we assume it's true that seraphs wouldn't be effective in that situation and would get picked off too quickly to do any appreciable damage to SMAC defenses, what we're left with is essentially a 50-supply, 4000 credit structure-bashing capital.
And I still don't think that where you have a couple DDS and a couple Assault Carriers all loaded down with seraphs that static defenses/a defensive fleet can do enough damage to prevent you from killing a fair number of SMACs. Eventually they'll wipe the floor with the seraphs and you'll be left with no strikecraft, but you'll do enough damage to the static defenses to be effective. If one seraph does around 11 dps, 40 squadrons x however many seraphs per squadron is probably enough to alpha multiple SMACs at once on the first pass, which is way easier and cheaper than building Adjudicators for the sole purpose of bashing SMACs.