One topic to tease out, here: there is a fundamental issue with size in D&D that is very difficult to resolve.
We want itty bitty fey folk and great big giant folk in our game. We want halflings and humans. We want size to matter.
The rules do not want size to matter. Size is part of power budget. A mechanically significant size decrease is a huge handicap across the power budget and a significant limiter on class choice. If, say, the best weapons are out of reach for you, you're pushed to make a character who doesn't use the best weapons, which means you can make a "bad choice" of being a Small character who is a Fighter, for instance. On the other side of the coin, if you're larger, and thus do more damage with weapons, you can make a "bad choice" of being anything other than the option that grants you Large status if you're a Fighter.
This is a solvable problem. The way WotC has currently chosen to solve that problem is to make size a temporary change with specific modifiers. Easier to account for in the power budget if it's not always on or if it has very specific results. Part of the problem there is that it fails to address the central fantasy - that of BEING a different size. You only get to be a different size sometimes. When it's not too inconvenient for the rules. In a certain, specific way that not everything is. For me, that's fundamentally unsatisfying. If I want to play a large character, I want that ALWAYS ON, I want to imagine my brick house of a character stooping to enter rooms and daintily picking up spoons. I want it to be something that I have to deal with as a player. If I don't care about playing a large character, then the appeal of growing large is purely a "is it time for my combat buff?" kind of question. Imagining them as being actually Large is secondary (at best). Fun sometimes, but not part of why I'm playing that character, really. Goliaths were always "as big as Medium gets," and that's fine. They don't need to also fill the niche of being Large, especially in such a half-hearted kind of way.
I'm sympathetic that it's hard to solve this problem for D&D in general. My preferred solution is typically to lean into abstract distances and Theater of the Mind combat, where things like reach and area of effect and occupied squares and whatnot disappear. D&D's legacy of tactical combat on a grid isn't going anywhere soon, so that's not really a viable path for the game officially.
I think they could lean into species being a more significant part of your character's power, which could give room for significant effects like large size, but it's looking like that's not the direction they chose, perhaps in a bid to make character creation easier (maybe?). Big effects like size differences, flight, or, idk, being a quadruped or a jellyfish, things that would disrupt some of the game's expectations of what an "adventurer" kind of looks like, these create weird and unknowable rules interactions that can be a big lift for various tables, and definitely aren't core-book material (don't want your giants and your fairies facing the newbies, really).
To me, giving Goliaths the ability to change size is a little bit like socks for Christmas. Okay, it's not nothing, it is useful, it might even be kind of cool and welcome, but it ain't really what I'm looking for under the tree, y'know?