what do you have in mind exactly when mentioning 'sidechains/poa' - do you, by any chance, mean an environment in which the client would have their dapp data on the private chain, with the nodes controlled by partners or a federated network of loosely-connected businesses, and a token, likely erc20 living on the ethereum public chain.

Yep. I believe the sidechain does not need to be publicly viewable. The authorities on the sidechain need to be able to send transactions on the main Ethereum chain though (assuming you are bridging a sidechain to Ethereum). Of course, an organisation could roll their own private instance of Ethereum or a POA network without bridging to a public network. Depends what on the use cases.

From GH: "parity-bridge is currently an ERC20 token contract on one ethereum-based blockchain that is backed by ether on another ethereum-based blockchain."

Then i imagine, some sort of 2 way proof of burn or at least an element of centralization in the form of script 'syncing' the two data repositoriels?

The sender will deposit ether into the bridge contract on Ethereum (mainnet) which then gives an ERC20 balance on the sidechain. You can withdraw ether at anytime.

i'd also like to understand if you guys at w3f have been thinking about clear , milestones migration paths towards a polkadot future, in order to limit the risk of accruing technical debt ?

There is a migration path. Adopting a poa network with a parity bridge is a good first step to getting ready for Polkadot. This will help to minimise "accruing technical debt", but as with anything software based there will always be some learning curve and testing required as new versions are released. Eventually the intention would be to migrate to Subtrate based chains.

A lot of tooling and development on Polkadot will be for Wasm chains (or rather Substrate chains, which allow for native and wasm), we hope they are faster than EVM chains. Devs can try enabling wasm on their chain and I believe they can do that already with Parity PoA chains. For example, devs can start practising with writing wasm smart contracts.