The first version lasts longer than you think
Nobody really tells you this, but the version you ship under pressure tends to stick. The one that went out quickly because there was a deadline, because engineering was waiting, because it worked well enough. It doesn't get replaced as often as you think it will.
When you're the only designer, there isn't much separation between a first pass and a final one. Decisions get made to keep things moving, and those decisions carry more weight than they seem to at the time.
Early on, that's easy to live with. Later, as the product grows, you start to feel what was never fully defined. Not as obvious mistakes; more like quiet constraints. Places where something almost works, but changing it means pulling on too many threads. By then, you're not really reworking the structure. You're working around it.
I haven't worked anywhere that had the time to slow down. Speed is part of the job. What I've learned is that structure is what makes speed sustainable. When the structure is clear, you're not relitigating the same questions every sprint. You're not patching over inconsistencies. You're building on something that holds.