A bridge, or underpass, will come with issues. Some people will be unable to manage steps or ramps, others will simply not wish to be delayed and will step off the kerb and cross the road as normal. Also, the segregation of home and away support would be nigh on impossible. So, cast aside the obvious problems of cost and logistical build and you have to say that a temporary closure of the road is the logical and safest way to allow folk to cross from Parkway/Water Eaton to the stadium.
My question to the planners is this; Which approach will those wishing to use the Water Eaton Park & Ride who arrive after the road closure use? Coming on the A34 (both directions) A40 and A44 will have to circumnavigate the stadium and approach from the Cutteslowe area - Banbury Road/Oxford Road. This is quite a diversion and for many that are unfamiliar with Oxford not an easy route (Think satnav directions and the sight of the stadium disappearing in one's rear view mirror!) There could also be a temptation to use Five Mile Drive as a cut through.
The solution could, maybe, be to use the Pear Tree Park & Ride and if the proposed "Shuttle Buses" come to fruition, on to the stadium without going near Oxford Road or Water Eaton. That would take a fair bit of communicating and again, those unfamiliar with Oxford might struggle. (Ok, the other answer is to arrive earlier, before the closure and enjoy the hospitality of the stadium which would be the Loungers choice)
As to the ridiculous suggestion from FoSB and cohorts that this amounts to a blockade of Kidlington, perhaps they should consider that this is all being done to reduce the use of private transport and to ensure the safety of those attending and, what they previously screamed in anguish, the need for anyone to park in their beloved village! In return they gain easy access to a state of the art sporting venue with hotel, restaurant, cafe and conference facilities, financial support for their existing sport facilities and teams/organisations and improved infrastructure with increased bus services and better access to Oxford via Parkway and Water Eaton. Not to mention employment and a boost to local business.