yeah but you do have a fair amount of flexibility as to where a x - Basel train is scheduled on the LGV Est to fit the gaps it needs to hit south of Strasbourg.
You need to look at the full complement of departures. Amazingly Strasbourg has a pretty damn good regional takt in readiness for the Deutschlandtakt. Selected departures from Strasbourg are as follows:
24/54 - stopping to Brumath
22/52 - TER to Offenburg
21/51 - fast TER to Basel
19/49 - regional TER to Brumath and Saverne (Nancy and or Metz)
A Paris - Strasbourg train would have half the train getting off and on at Strasbourg so you need more than the standard 3-minute (TGV standard) dwell. Strasbourg is a big station so you want to give passengers more than 5 minutes to change onto the first departure in the pattern. The Deutschlandtakt suggests a xx43 (and 13) arrival from Paris Est, which would allow a 6-minute interchange onto the xx19/49 path to Brumath. I don't see any good reason to deviate from that.
Remember any Paris - Strasbourg train will be for Paris - Strasbourg and surrounding communities first and foremost and an additional Paris - Basel train as a bonus on top of the core service via Dijon. It should serve the takt first and raw journey time second.
By my estimation the Paris - Basel journey time could be 3h02m:
xx28 Paris Est depart
xx13/19 Strasbourg
xx30 Basel SBB
If it's an Ilisto service being extended, then the Paris departure would be earlier because Ilisto plans to call at all 3 of the TGV-Parkway stations.
Of course the UK might one day join Schengen.
It now has more migrants crossing in boats than ever present themselves via Europe at stations or airports. So usual argument for keeping border with mainland Europe is rather weak, and when they do arrive we don't go in for Trump style deportations.
There are already 4 non-EU countries which have joined, Liechtenstein, Iceland, Norway, Switzerland. Only 2 EU countries have not joined, Ireland which is happy to, but wants to maintain open borders with Northern Ireland, and Cyprus (which is applying). Checks at land borders of two recent joiners (Bulgaria and Romania) were dropped from 1st Jan 2025
Schengen isn't only the mainland Europe, it includes many of UKs common holiday destinations, Canary Islands, Malta, Madeira, Greek Islands, Corsica, Azores.
The new entry-exit system effectively is e-gate booths which take on average 20 seconds per passenger. Thus a 700 seat train every 30 minutes needs 6-7 e-gates (actually bit more as passengers won't arrive evenly spaced, and could be failed gate awaiting repair). If trains are less frequent and passengers joining at multiple locations then many of these secure platforms might just have 3 or 4 e-gates, and just 2 border staff members on duty ahead of departure. It is not clear to me who would pay for these.
Oh I think the UK's current attitude towards Europe is completely retarded. All these unnecessary trade barriers across the channel is having a real depressing effect on our productivity and tax revenue just when our economy is in the doldrums. But I'm not going to hold out for any political miracle, at least not in the short term ...
Hopefully e-gates will reduce the space requirement at St Pancras and the staffing levels in the Dutch stations and make these logistical and funding constraints easier to manage.
The problem with any further upgrades on the Belgian side of the border is that they involve the Belgian/Walloon government having to pay (apparently the Belgian federal government has a budget deficit at the moment), and the Luxembourg government has a significant surplus.
I am aware that the topography of a route from Lux City to the Belgian border isn't great for a HSL line, but the responsible government has more ability to pay for it at the end of the day.
Also, you could make use of the unused underground station box at Lux Airport for part of the first section of the route; with the wide range of destinations it offers in comparison to most other nearby airports, it has the potential to attract some additional passengers looking for a decent hub.
(Liege Airport has basically nothing for passengers - apart from a few TUI routes, only a few routes operate year round from Maastricht, Charleroi is a decent low-cost hub, but mostly European flights etc)
And yet all that Luxemburg is prepared to sign up to is the jointly fund the modest Arlon - Namur upgrade, and is wanting EU funding. The kind of generosity you envisage from Luxemburg is simply not forthcoming.
At the end of the day what you are advocating is akin to a Swansea - Chester high-speed tunnel that would carry fresh air year round.