Onboarding / offboarding example not entirely accurate. The UNI and AAVE token are both available to swap through the Quickswap DEX on Polygon. There would be no need to bridge between various blockchains in order to perform simple swaps. Don't get me wrong through, liquidity fragmentation is a legitimate scaling issue.
Yeah probably not the best example to highlight — although Uniswap will be exclusively on Optimism, even if the UNI token has been bridged to Polygon so the issues remain