Is Exchange Server 2010 SP3 still necessary after all mailboxes have been migrated to O365?

We are currently in a Hybrid setup using Office 365 with Exchange 2010 SP3. Once all mailboxes are in the Cloud and On Prem Distribution Lists have been recreated in the Cloud, is it necessary to keep any footprint of an Exchange server On Prem? We are using AD Connect to sync new users to the Cloud. Management still thinks an On Prem Exchange server is still and will always be necessary in the Hybrid setup. Your thoughts please.

Anthony K O365Asked:
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Its recommended to keep exchange server onpremise without mailboxes so that user management tasks can be controlled from exchange server
Exchange server will ensure that every proxy address you sync would be unique and there won't be any duplication issue
further Microsoft do not recommend editing exchange specific attributes manually and it should be done via proper tool like exchange management tools
So exchange server which remain on premise would be nothing but management server for AD accounts and used to manage accounts exchange attributes MS also provides you separate hybrid to key to use that server as hybrid server, its expected that you should not host any mailbox on this server
Anthony K O365Author Commented:
When you say a MS provides a key to use for the Hybrid server, do you mean an Exchange Server being used as the AD Connect Server?
No man !

What I am saying MS don't want you to invest separate license to keep exchange server onpremise, within your E3 / E4 / E5 subscription, they provide you exchange server hybrid key which can be installed on exchange server
In simple words, today you might have exchange 2010 hybrid server which is already licensed
But tomorrow if you want to install 2016 / 19 exchange server as hybrid management server, you don't need to purchase it, you will get key from Microsoft for same as part of your enterprise O365 subscription

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Anthony K O365Author Commented:
This was very helpful.
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