I must be the only person who doesnt feel he falls off late game and gets totally ignored.
His Q provides massive zoning and one good Q is basically a malphite ult in what it provides. Similarly, if they interrupted it then they wasted a cc ability on you instead of your team, which is still a success from a tank standpoint. His W is a large aoe %HP shield OR %HP nuke (dont bet on getting both late game), his E shreds 20% armor of the first thing it hits, literally giving your ADC basically the armor shred cleaver will give in a fight on avg, instantly on someone probably on the enemy fronline. His ult is godlike follow up/peel cc with its point blank applications, and long range is a good engage, though trades that for less reliability. he does however have the reliability issue though of being mostly skillshots, but if aimed well is a god.
Im sorta confused as to why his Q being interrupted or any specific part of his kit is what hurts him lategame and doesnt trouble him early or midgame. His base dmg and utility are amazing at all points of the game, and even if its not his best dmg item titanic does give the potential to get that active auto on a squishy who'll say goodbye to a third of their hp. his HP stacking means he can get only spirit visage and sunfire as his hp giving items and can stack raw resistance items like thornmail, zzrot, and iceborne and/or frozen heart and still come out with 4k hp being literally the tankiest champion in the game from a raw resists/hp standpoint, and has a massive shield too.
The only reasons he may feel he falls off late is he lacks some of the backline access of other tanks or fighters like zac, malphite, jarvan, maokai, poppy, or vi. He instead is in the group of something like amumu or sejuani who, unless they land a god ult, get a great flank, someone mispositions, will often end up ccing entire frontlines and provide indirect backline access this way.
Also late game ADCs and DPS mages will burn down all in their path given enough time not just sion, and even the burst mages or assasins will make you wonder where a good chunk of hp disappeared to along with maybe some of your teammates. this often means sion might need to play more of the role of a peeling tank, rather than engaging so that his backline is alive long enough to get off dmg. Also worth noting that he is one of the best at ccing frontlines of all tanks, and generally has one of the best laning phases of all top laners too.
(if you are wondering why sions best dmg item isnt titanic, its because due to the nature of his Q, which you probably want 30-40% cdr to be up all the time to zone with or now and then land those juicy knockups, he only gets off a few autos and doesnt get the most of its on hit as say some tanks that engage and then sorta just walk around for a bit like malphite (also a reason Iceborne gauntlet is good on him for the spell weaving effect) and better damage items I recommend Maw for the armor pen for his Q and R, or potentially liandries or abyssal for magic pen for his W or E and all these items also give decent defensive stats)