to answer the OP's question....

no it wont empty the mempool.. infact it will fill the mempool

the only way to utilise the "weight" is to first move funds out of old transaction format (legacy keypairs) and then spend again in new segwit transactions (sw keypairs) then and only then would the segwit transaction utilise the weight area.

the issue with this is if everyone was to move to use segwit keys to even get close to a 2.1mb weight estimate.. everyone has to move their funds out of old legacy keypairs..

there are millions of UTXO's using legacy keypairs, meaning alot of spending needs to occur BEFORE its treated as segwit based funds..