To make rETH holders whole, you need to either (a) send ETH to the rETH contract, or (b) send rETH to a burn address. The former is pretty straightforward to implement (don’t even need smart contract changes). Taking from ETH rewards is possible, but would require entirely new flows (we already have an RPL auction, so it’s a lower implementation bar). Taking from ETH bond is pretty rough imo - it makes security discussions more difficult if we have non-security ways to eat into ETH bond.
The RPL “route rewards to stakers” vs “burn” is a choice about the point. It’s presented as countering the impact of RPL auctioning, which damages all holders (not just stakers), which matches “burn” better.