The usage vs St Pancras would be negligible and the overhead in staffing it would be enormous.
Juxtaposed controls are an anachronism for rail, they were intended for conducting dog searches of HGVs boarding ferries.
It is the EU adding friction at the border removing the ability to travel at short notice.
UK didn't bother with passport stamping and ROI won't be involved in the new arrangements despite being outside Schengen.
It is unclear what problem the new arrangements are designed to fix. API introduced 15+ years ago was designed precisely to stop anyone on an 'exclude' list even getting into the air.
Removing the ability to travel at short notice will not matter to many passengers in reality. Once the EES and ETIAS have bedded in it isn’t likely that many people will be turning up at St Pancras at short notice without being enrolled on EES and having ETIAS. Very few who have not entered Schengen for three or more years are likely to suddenly need to take the next Eurostar!
EES is designed to fix the need of border officers of Schengen countries to manually count days using passport stamps. ETIAS is designed to curb travel of risk individuals. At some point it should enable visa requirements to be waived for some nationalities and individuals. Having a basic background check (UK ETA) has enabled the UK to drop full visas for Jordanians. Canada allows citizens of growing list of countries to just use electronic travel authorisation if they have had a Canadian or American visa in last 10 years.
The talk of capacity restraints at St Pancras is simply because of money, nothing more. There's definitely space to add at least another train worth of passengers with only minor remodelling required.
Fixed that for you. Eurostar has form for trying to use its popularity with Europhiles to try to get UK taxpayers to subidise their business e.g. during the pandemic they expected better terms for financial support than offered to airlines. The appeared to go through a stage of being unwilling to pay for any extra space at St Pancras but have finally leased more. If they want to expand they will need to lease even more.