My right shift key on thinkpad is full size and i've got full size up/down arrow buttons too (on a 12.5 inch laptop). So it definitely is doable and there is no need for tradeoffs.
This, in turn, is a trade-off for manufacturing. It will increase the cost of everything slightly. The keyboard unit itself as it will be larger, maye be harder to produce. Machining or producing the chassis now requires accounting for an additional cut.
This is all solvable, yes. Though it is harder for smaller scale productions. It is likely they are working with vendors that, themselves, only have "entirely rectangular" keyboards as this is what the market ends up wanting for cost reasons.