English and German have only two tenses (the present and the past) that are formed by inflection, all the others are formed using helping verbs, as is the conditional mood. In the Romance languages most tenses are formed by inflection, and so is the conditional mood. Years ago I read that the relative simplicity of the verb in the Germanic languages dates back to Proto-Germanic, and there is a theory that Proto-Germanic was the result of a creolization of Proto-Indoeuropean. This would fit in with the idea of a Germanic substrate of words not found in other branches of IE. More recently I read that the complex verb structure was a late development in Proto-IE; it is not found in Hittite, which is believed to have separated from Proto-IE at a very early date. Is it possible that Proto-Germanic also separated early from Proto-IE, and that it not need to lose the complex verb structures because it never had them to begin with?
Ringe (2006) in From Proto-Indo-European to Proto-Germanic discusses verb inflection and sub-grouping on p. 4-6. He states the consensus position that Anatolian split off first, leaving behind what he terms "North IE", then Tocharian, leaving behind "West IE", followed by Italic and Celtic, leaving behind Central European – the rest of IE – considering it "possible that Indo-Iranian, Balto-Slavic, and Germanic were parts of a dialect chain at a very early date". He then states w.r.t. the verb that
Note the implications of this phylogeny for the reconstruction of the PIE verb. The Cowgill–Rix verb is a reasonable reconstruction of the system for Proto-West IE, and can even account for most of the Proto-North IE system; it is only for ‘PIE proper’ that it is clearly inadequate.
That at least provides a decent basis for believing that Germanic is not an outlier as far as verb inflection is concerned. See pp. 27 ff. for discussion of the details of the Cowgill-Rix system, and its development into Germanic starting at p. 151. I think the problem is that the clues to the earlier state are not thick on the ground as they are in Sanskrit.
Is it possible that Proto-Germanic also separated early from Proto-IE, and that it not need to lose the complex verb structures because it never had them to begin with?
That's my vibe. It chimes well with the Glottalic Theory of Ivanov and Gamkrelidze, who saw German closer to Anatolian – the oldest split – and Armenian, on phonological grounds, but that was not widely accepted.
It is true that they sometimes speak of the Cogwill-Rix verb or Indo-Greek. However, the morphology is not limited to that.
An early split makes sense in the monotonic branching tree model, but it is not clear if the model makes sense in reality. The requirement of more comparanda in number than the reconstruction it results in will lead to ever bigger grouping and proportionally less precision.
There are too many indications of recent innovations alover the place. Old age is not necessarily the default assumption.