That is not quite true in the new timetable.
The key daytime sailings for both Stena and Irish Ferries have Avanti connections:
The 14:05 Irish Ferries Ulysses sailing from Holyhead to Dublin connects out of the 09:10 Euston-Holyhead, and the 14:45 Stena Adventurer sailing from Holyhead to Dublin connects out of the new 10:10 Euston-Holyhead service.
The 08:05 Irish Ferries Ulysses sailing from Dublin to Holyhead and the 08:15 Stena Adventurer sailing from Dublin both connect into the 12:53 Holyhead-Crewe and onward to London.
The overnight market, using the sailings currently operated by those two ships died years ago, slain by the arrival of Ryanair. That market is frankly dead as a dodo and is unlikely to return.
There is a market for Sail/Rail but it is more focussed on those key daytime sailings.
The other Irish Ferries sailings operated by the Epsilon doesn't take foot passengers, so they can be eliminated from the list.
The overnight Stena Estrid sailing from Dublin at 02:15 does not have a bus connection into it at Dublin Port, but does connect into the 06:55 Avanti service to Crewe, and the 09:00 sailing to Dublin connects out of the 06:25 Avanti service from Crewe.
The afternoon 14:45 Stena Estrid sailing from Dublin is one of the few that has a TfW connection, but that has an Avanti service behind it from Bangor onwards to Crewe, and while the 20:30 sailing to Dublin connects out of the 15:06 TfW service from Birmingham International, that sailing has no public transport connections at Dublin port arriving there around midnight, which makes it rather unattractive.
The Irish Ferries SWIFT sailings do all connect into/out of TfW services, although one of those is operated by a Mark 4 set.
Just to be clear the new timetable has four workings across the North Wales coast to London Euston:
04:48 Holyhead-Euston
05:51 Holyhead-Euston
13:57 Holyhead-Euston
14:50 Holyhead-Euston
There are a further three services from Holyhead to Crewe, and one from Bangor to Crewe.
It has five in the opposite direction:
09:10 Euston-Holyhead
10:10 Euston-Holyhead
16:10 Euston-Bangor
18:10 Euston-Holyhead
19:10 Euston-Holyhead
There are a further two services from Crewe to Holyhead.