Well FWIW I would say that it normally takes a cadence onto the home chord of the new tonal centre, but the fact that the first chord of a progression is a dominant seventh is not enough to make it a cadence – it is also necessary IMO that the chord to which it resolves is happy to stay put (which pretty much rules out another dominant seventh) and that it occurs at the end of a phrase (which pretty much rules out the chords in the middle of a chain).
If you ask yourself what note you would expect the piece to end on, if it were to end at the first possible opportunity, that note is what you perceive as the tonal centre at that point (it may be better to call it the root of the chord that defines the tonal centre). If there is no answer to that question, we may be in freefall – a chain of dominant sevenths might create that effect, but then again we may only be going round the same arc of the circle of fifths that defines the scale (if you look at any 7-note arc, you will find that the notes it includes always make a natural scale, i.e. a major scale or any of its modes, including the natural minor). Going round the arc in that way is really the harmonic equivalent of playing a scale, so doesn’t really suggest a change of key IMO. If we are going to stay with the circle of fifths for the length of the arc but still get back where we started, one of the chords will have to be different – otherwise the chain will overshoot by a semitone and will then continue on around the circle. In the end the reason for this is the same as the reason for the semitones in the scale and many of the issues around temperament – the fifth does not fit neatly into the octave.