I would have used estaba in a real conversation but despite using spanish as my primary spoken language most days I am still not sure on certain situations using subjunctive tenses.
I intially used estaba (or at least that was what I meant to write), but then I remembered that "(no) es probable que ..." and "(no) es posible que ..." are impersonal observations and require the subjunctive tense. If I recall correctly (and I am certainly nowhere near an expert in the Spanish language), "es posible que ..." and "es probable que ..." are special in that their negation also uses the subjunctive tense.
Of course, it would certainly make sense if many forms of the subjunctive tense are not used often in conversation since the future subjunctive has essentially died in the Spanish language (which indicates that the subjunctive tense as a whole might be losing importance). This wouldn't be surprising since the subjunctive tense in other Romantic languages is smaller than that of Spanish and since several languages, including English, have massively reduced the usage of the subjunctive tense. Or it might be that the importance of the subjunctive tense varies by place so I was taught to use the subjunctive just like I was taught the vosotros conjugations in case I needed to use it somewhere.