If I have to just say {wa'leS SIStaH 'e' vI'aq} or {wa'leS SIS 'e' vI'aq} instead, since verbs taking SAO can’t also have a VS7, then the information that {'aq} is completed has been lost, and if it can be lost here, why not somewhere else? Certainly, the aspect of the verb taking the SAO doesn’t come from the verb in the SAO, since {wa'leS SISpu' 'e' vI'aq} sounds like the prediction is that it will stop raining tomorrow, when what I really want to say is that I finished predicting that it will rain tomorrow. (I’m now fairly convinced that you do indeed need to use aspect markers when the meaning calls for them, so I’m not holding this up as an argument to say they you don’t; I’m just trying to understand the ramifications of this restriction more fully.)
It's true that sometimes the arbitrary limitations of the grammar mean that sometimes you have to find other ways to say what you want, when violating the rule would give you exactly what you want. I can't tell you how many times a thorny problem would have been solved by being able to put -ghach on a verb with no suffixes, or being able to put something other than noun phrases in the comparative construction, or being able to use -laH and -lu' at the same time without slang, or, in this case, being able to specify aspect on the second verb of a sentence-as-object. You just have to look elsewhere.
Okrand has stated that he regrets making -laH and -lu' mutually incompatible. The solution is illustrated by his translations of Kahless the Unforgettable: qeylIS lIjlaHbe'bogh vay'; qeylIS lIjlaHbogh pagh. This is how you have to get around these limitations.
You can't say wa'leS SIStaH 'e' vI'aqpu', but you can say wa'leS SIStaH; muD Dotlhvetlh vI'aqpu'. It's awkward, but it's awkward because you're insisting on focusing on the completion of the prediction. I get the feeling, from Okrand's translations as well as the text in TKD, that the second sentence of a sentence-as-object is supposed to be a fairly lightweight thing, not meant to carry the bulk of the meaning: "['e' and net] are used primarily, though not exclusively, with verbs of thinking or observation (such as know, see)." When we go far afield from that, we strain the ability of the language to deliver the intended meaning.
-- SuStel http://trimboli.name