What makes Possente Spirto impossible is that there are extremely long and sustained musical phrases, which ought to be sung without taking a breath - and therefore must be done at a reasonably quick tempo; also if performed too slowly the music loses cohesion, and becomes dull.
However, embedded within these long phrases are decorations - runs (short notes, going up and/or down the scale) and repeated notes (sometimes called Monteverdi 'trills', but not really so, because a trill is a rapid alternation of two notes). These decorations are extremely difficult to articulate at the necessary speed, to differentiate each individual note clearly from the notes on either side.
And especially the machine-gun-like rapid-fire repetition of single notes (i.e. that 'Monteverdi trill') is something which singers are nowadays simply never required to do - and hardly any singers can get anywhere near to achieving it; but instead just slur over the repetitions; with a great reduction in the dramatic power.
Here the aria is done by Anthony Rolfe Johnson in a very highly-regarded performance (with probably the best modern conductor of this music) which you can follow on the score; however - if you do this - you can see quite clearly that ARJ is just-not singing all the notes of the rapid passages - nor is he separating the rapid repeated notes.
Rogers voice was neither loud, nor (to my ear) was it particularly sweet-toned - however, by attacking the rapid decorations (with an almost hair-raising effectiveness!), he achieves a dramatic quality (in the right way) which is overall more effective than his many later rivals.
Judge for yourself:
Anyway - this first great tenor aria is perhaps the only one that is also impossible; at any rate it seems very unlikely that there will ever (ever again?) be a tenor who has all the qualities of tone, power and agility necessary to sing Possente Spirto as well as it might potentially be sung.
