Oh absolutely, it's impossible to know. Lucky Bancho's Lodestone scrapes put the figure at around 25% for lv60 healers on average plus however many people play BLM+SMN at that level, and they aren't exactly unpopular DPS jobs. I'm sure there are plenty of career SCHs who never touch SMN and vice versa, but then there are also people like me who level MCH to 60 and forget about it thereafter (after buying its eso gear T_T). Two non-casters are locked behind HW compared to only one caster, and one other non-caster offers no cross-class incentives and is only available after you level a different class first, perhaps becoming attached. It's not really that easy to come up with a fair rule based on ease of access to the job.
Even if equal numbers of people main each job at endgame - we know this is untrue but I'm going to assume it's S-E's philosophical ideal when balancing things because that makes sense - that yields 38.5% as the figure for people sharing a single gear model (more if you weight the HW jobs differently to reflect the relative difficulty in obtaining them or try to balance for the free second caster all ACNs have access to, though I'll keep it simple as the number is high enough as it is). Irrespective of whether it's their main job, or whether it's their secondary job, or whether they just levelled it for PVP and don't use it to raid, five out of thirteen jobs sharing a single model represents a large number of players all wearing identical gear. Now that five different jobs are sharing rather than four, it's more glaring than it was back in 2.x.
Two different cloth models in future, please. If they can only design a certain number of sets I'm more than happy to share with MNK and be a half-naked martial arts BLM now and then for variety, or a leather-clad BRD wannabe with frills and spikes. Either gives way more glamour possibilities than 'black version of the healer robe'.