An extension wouldn't provide more amenities, unless you are proposing a full stand rebuild, adding seats, with the design of BMD doesn't necessarily mean adding provision of amenities as the two structures are for all intents and purposes separate from each other from where the upper bowl rises above the brickwork. 'Simply' adding seats would require a partial destruction of all existing (or what would be existing) concourse areas, addition of another concourse level to provide these additional amenities you mention, leading to full roof structure remodel. Not to mention the piling that would be needed for the additional bowl structure.
Also, the canal isn't 'aesthetics' there is avital need for the waterway connection to be maintained in order to maintain teh living nature of the docks, and its inclusion in any design was a fundamental part of any agreement on both lease and planning. It may be be maintained as an underground connection if aesthetics aren't important, but the physical link must be maintained.
Even though we lease the land, we are still under the restrictions of the regional SRF (Strategic Regeneration Framework) that we were subject to as part of the planning.
As much as we might hate to admit it, from the moment that BMD is opened, Anfield would be easier, and less disruptive/destructive to expand further.