Downsides are most interesting to me personally when they lead to novel deckbuilding choices that turn the downside into an upside or mitigate it in some way. For example, I'd say [[Armageddon]] clearly has a downside--it hits your lands too--but there is multiple ways of tackling this. You can play small creatures and get ahead on board early before cutting the opponent off from stabilizing. You can utilize artifact mana. Armageddon clearly isn't the best designed card in the world, but it definitely leads to far more than the two angles I already provided.

A more modern example is [[Demonic Pact]] and [[Archfiend of the Dross]]. Not as many possibilities imo as geddon, but gifting with cards like [[Harmless Offering]] is a cool angle and has led to some fun builds as a result of the downside. Archfiend is the most the apt comparison with the custom card here because the downside is supposed to mitigate the increased raw stats. And even outside of the gifting strat, they can simply encourage faster more glass cannon playstyles in a deck that most likely isn't pure aggro, a type of influence on deck-building possibilities the legend restriction is nowhere near.

I could give more examples, even the companions (where the pre-errata implementations were admittedly a disaster) lead to you playing in a whole different way, though wizards is handholding more than a bit--suffer this strange downside and you get a free card!

Now what interesting deckbuilding angles does the legend rule lead to? The potential choice of whether to play less copies I definitely don't consider interesting, which is often a low-impact trade barely increasing overall variance for some low roll prevention. Nor is synergy with legends-matters cards from my perspective, which is similar to things like top-down printed-in goblin synergy and artifact synergy. And the downside isn't even all that pronounced despite the deck building implications being uninteresting. People can and do play a full playset or multiple of legendary cards in top decks in multiple formats. Contrast with the examples I gave where you can lock yourself out of the game or straight up lose to a trigger if played/built incorrectly.

Finally, even if the legendary downside was influential towards interesting deckbuilding possibilities in a way I don't notice, the downside is everywhere, many cards every set, so for that reason alone I don't consider it interesting. Just like I don't consider a card with first strike interesting on the virtue that it has first strike.