Not really true. Or rather…
Underlying systems basically function as constraints to the complexity of the game. The game can’t really become more complex just because newer heroes are introduced (but there is an element of “knowledge” here that underlies this, but that’s a separate topic). Just because a game has 40 heroes, or 400 heroes (like a typical ‘gacha’ game would) wouldn’t be making the game more complex so long as the core functions of the game remain the same set of staples.
In Overwatch’s case, this basically boils down to having a Shift (usually movement), an E ability (temporary, active, aggressive, utilitarian, supplementary), and an Ultimate (Q). It won’t really get more complex than that.
With that said, however, new mechanics introduced into the game further the complexity of the game. While Overwatch had several staples already, from damage over time effects, to debuffs, skill-shots, immortalities/invincibilities and more, it’s apparent that many players aren’t even aware of them or the ability to clear them has always been available. This is largely because Overwatch does an incredibly poor job of explaining or communicating this,
Further, once considered staples of certain roles and “plausible rules” of each role were thrown out the window with each new hero introduction. As an example, think of how barriers were once exclusively the domain the the Tank-role, until Brigitte received one. Or how limited crowd control was in the Support-role until Ana, Brigitte, were introduced and Lucio’s own was buffed. Think of how supportive and utilitarian the old Defense-role was when Torbjorn used to offer armor packs, and Symmetra used to provide shield health and Teleporter. Or how Mei still offers accessibility through the use of Ice Wall for less mobile heroes.
With a constantly changing baseline set of core rules, Overwatch’s own complexity continued to grow, and what was once perceived as a typical template for a number of heroes, is now anyone’s guess. No one really expected Wrecking Ball to be as mobile or one of the fastest heroes in the game when past Tanks never had the kind of speed or mobility that he offered. Nor did anyone expect that a Tank could provide Overhealth or Speed boosting until recent Tank releases. There has yet to be a new Damage-role hero introduced that offers any new mechanics or crosses into other roles that the Tanks and Supports have. All the “supportive” functions that some Damage-role heroes have are still retained by heroes that were included in the launch of the game, and much of their design seems incredibly outdated by ever shifting direction Overwatch’s new heroes; both from OW1 and OW2.
Take Illari, the first Support hero to have an ultimate designed entirely for dealing damage; Moira originally crossed that line here, but Coalescence offered more healing and its damage is entirely supplementary, if not downright ignored.
This constant shifting of the established “rules” are the reasons for why Overwatch can be as complex as it is, and why newcomers have difficulty coming to grips with the game. Even players with more time or established reputations with the game, still fail to understand some basic mechanical constants; because there’s no consistency.
Look at FF14 and how it has more than 20 Jobs in the game. Despite the fact that there’s more new Jobs in the game that in its first years, the game maintains the same complexity it always had and established since Stormblood. Because FF14 doesn’t change the fundamental baselines of its systems. New abilities are introduced and removed and consistently remain within 24 to 26 buttons; more recent changes with this expansion have expanded some baseline principles, but without making it overly complex to understand.
Even the various Jobs have established principles when it comes to gearing (something that WoW once coined with its gearing of Plate, Leather, Mail, and Cloth). And many Jobs - while individually complex - still adhere to some very specific standards. Even the newest Pictomancer and Viper jobs bear some resemblances to the existing roster, while also carrying a lot of unique elements of their own.
Overwatch doesn’t really do this, as illustrated above. That’s why the game can be so complex. Not every Damage-role hero has a damaging ability; Tracer doesn’t, Mei doesn’t, Reaper doesn’t. You may not even be aware that many of the “bulky” CQC heroes; Reaper, Mei, Venture, and OW1 Doomfist, not only had higher health than the rest of their team, they also have individual mechanics to sustain or heal themselves with. Mei’s Cryo-Freeze, Reaper’s Reaping, Venture’s shield generation, which he steals/borrows from OW1’s Doomfist’s Defense Passive. Because this view runs counter to why Bastion and Torbjorn have higher health, and why half of Symmetra’s health is shield health.
We have players here that complain that Juno doesn’t have a way to Self-Regenerate as consistent with other Supports; still completely unaware of the fact that she does have Self-Regeneration or healing through a bulk of her health being shield health. Because that runs counter to the consistency of other Supports having active healing (excluding Zenyatta, Brigitte, Mercy) through a cooldown ability.
It is these combinations of poor communication, lack of consistent principles, and lack of tutorials to explain how thing work that makes Overwatch complex. No matter how many heroes are in the game, you’re still locked to 1 Tank, 2 Damage and 2 Supports, so the combinations aren’t limitless and bound by a specific set of principled rules and expectations. Until Blizzard decides that those expectations don’t apply to the next hero.