I don't think addon usage is indicative of something being too difficult, really. Ultima Weapon Ultimate is considered one of the easiest, if not the easiest, Ultimate to do and people still utilize third party tools in that fight - with or without VOIP involved. People will, generally, always use something they find to help them to win at something - be it programs that automatically assign roles in the heat of the moment, or programs that tell you precisely where to go, or programs that do personal shotcalling and so on. There's no level of content that's free of people utilizing tools to make it easier, for one reason or another, and I wouldn't say difficulty is necessarily correlated with accessibility - accessibility is making sure a designed fight operates as intended for those with disabilities, such as colorblind/blindness (pairing image with indicator/telegraph and audio queues, for example); trying to keep responsiveness tighter so someone can play a class effectively despite a higher ping/weaker connection inherent to their ISP/Location/Set up etc... or making sure the game can operate on devices that are there to allow people to play the game with other physical disabilities (there's a plethora of specialized keyboards, mice, controllers, control schemes/set ups and other devices in this vein). And, I'm sure, a lot of other considerations not tied in with the intended difficulty of the experience (although, I suppose, that description relates to: unintended difficulty, as someone who can't see what they're supposed to see are not experiencing the fight's design as intended -- and not out of a lack of attention, but in a genuine "it's invisible"/"it's non-existent" -- I had some issue with this, as well, with Midgardsormr in Omegascape back when, with the shield to pick up - me not being able to see it on the ground isn't an intended factor of difficulty).
A fight can be easy, and inaccessible (I found the Phoenix Mage in Bozja particularly egregious for me, since I had to shift the colorblind settings until everything was a mountain dew green to see the birds on the sides of the arena and, without that - which was headache inducing - they were basically invisible against the backdrop/ground for me). And just because, for example, I find something difficult by the basis of it being designed for that experience, doesn't mean that because I find it hard it's "inaccessible" really: it'd mean I'd have to put in more time to clear/beat it (which, time isn't something everyone has access to/wants to spend time on that, but that's less accessibility and more dealing with factors outside of reasonable control for an intended experience).
I don't really think Ultimates must be easier/harder - DSR is the one most people I know cite as their favorite, even after running the gambit, and that one is one of the more difficult ones from design/current scaling from what I hear with mixed opinions (in my experience with what people who've done it say) on TOP, although most I know say TOP was a lot less fun for them.
They just need to stay at a difficulty level for the expected target audience to reasonably clear, reasonably being (broadly) 3-6mos, as that tends to be the trend for most who do Ultimate in a non-competitive capacity, from what I've seen (Some take longer, some shorter, and if someone's raiding over 20hrs a week to clear it as soon as they can, I wouldn't necessarily classify them in the broader audience I'm referring to). And the design just needs to feel fun, and rewarding. No matter what's done with Ultimates, there'll always be various third party tools utilized to make individual experiences easier, or group experiences.
Personally speaking, I don't really care about third party tools unless they began to impact my experience (ie: if the dev team decided to design around them, leading to the potential situation where a player would suddenly /require/ them to participate in the content, or by nerfing the content down in hopes to curb the tool usage). But, the main factor as to why I don't really do Ultimates is more related to time (between finding/putting a group together, to scheduling, to actual time in etc...).