It's absolutely usual for railways to open without a grand terminus.
Most of the early railways had severe financial problems simply building the approved railway route, and didn't have the means to build a proper terminus.
Euston itself was like that in 1837, while Robert Stephenson struggled with cost overruns and delays along the line of route to Birmingham, eg at Kilsby tunnel.
The initial estimate for the London & Birmingham was £2.5m, but it cost £5 million to get the line open.
Sounds a bit like déjà vu to me, but with the numbers of pounds multiplied by 10,000.
Arguably Euston never was the terminus the L&B/LNWR/LMS really wanted for the "Premier Line".
Most grand UK terminals were built at a later stage of development and expansion when revenue was flowing, and Euston never quite got it right.
Manchester Central is another example of a terminus which never got rid of its "temporary" wooden structures before it was abandoned by rail in 1969.
The Channel Tunnel never got round to building the intended architecturally impressive portals (they are pretty much invisible anyway).
There's general silence about the Birmingham end of Phase 1, but it wouldn't surprise me if the construction of the station at Curzon St was phased in some way to reduce the rate of spend until the network grows after several years of operation at low capacity.
Silence also about Birmingham Interchange, another station with an uncertain business case, with no interconnections like OOC.
Old Oak Common is the one site that can't be skimped, or we will throttle the system for decades.