Why are people tripping over the microtransactions? Are you really that deluded that you think Rockstar wouldn't monetize GTA Online somehow? This is probably the best case scenario on how they chose to go about it. Microtransactions in this instance is a GOOD thing.
That means that only the whales will pay for GTA Online, and Rockstar can keep the core updates free. The alternative would either be releasing the upgrade packs as DLC with a price tag, or making you pay a monthly fee for GTA Online down the road.
I suspect eventually they'll still add DLC packs that you have to pay for, things that add maps and new story NPCs like what Lamar and that "Keep your mouth shut about me you hear?" guy do in GTA Online to further that character's story, but other than that, content updates will probably remain free.