No. Castillian won because of pre-industial demographics. That made it the language of the court, not the other way aroud. In fact Galician (an early Portuguese) was the poets language in the Castillan court almost until the unification itself. Many nations had French as a court language for centuries, and yet they did not adopt french.
Portugal and Aragon were maritime, commercial kimdoms centered around its seaports (Lisbon, Porto, Barcelona, Valencia). Castille was made of huge river valleys than grew loads of grain and sheeps -and grain and woold were in fact two of pre-industrial Europe mayor industries. It did not have many mayor cities, but had dozens of small ones. Castille had the other four nations (Aragon, Portugal, Navarre and Granada) beat both by wealth and population -it may even beat all four of then together.
It wasnt until the industrial age when demographics changed in Europe, and wealth moved from wool and agriculture in the country to the industrialiced areas around the cities, and great industrial centers developed from minor towns (Manchester, Bilbao). But by then Spanish was the Castillian language, and Catalonian, Galician and Basque were on the wane (while Catalonian managed to survive with some health, until the advent of democracy in 1978, and the establishment of bilingual education on those regions than restored their ancestral tongues, Galician and Basque had turned into insolated rural languages than had probably a couple generations left)
Wheter Spain is ruled from Castille, Aragon or Portugal, a Castillenizacion in a couple centuries is almost unavoidable, unles you get a completely radical POD in, say, the War of the Spanish sucession or the Napoleonic wars. Until well into the 18th century, european nations did not have even the concept of an "official language", and all nations had quite a few regional tongues.
I have a few criticisms of your argument: it starts with Spain as ruled from Castile. Or from anywhere.
When Spain was a union of the crowns, rather than a centralized state, Castilianization was limited to the courts themselves and to the aspiring classes. Volume of speakers was relatively few in number.
It is the constitution of Spain as a single crown, with one set of laws and customs, where languages other than Castilian are, first proscribed, then actively prosecuted. Still, until universal primary education is made obligatory with the Moyano laws of 1857 (previous ordinances already set punishments for those using anything else than Castilian in schools) language change by the population of the periphery is still negligible.
The concept of an official language as a de facto institution -by banning the rest- is earlier than the late 18th century. The pioneers were in fact, Bourbon France and Spain. (I need to point out here that Philip the V Bourbon did not bring his court from France, it did come to a Castilian court which had rooted for him in the Succession Wars.)
The problem with adscribing the advance of Spanish to market forces and demographics is that it makes the expansion of Spanish look like a historical inevitability, when it ignores the most important factor: imposition from above is the way things have been done in Spain for a long time.
If we imagine *Spain as ruled by a Portuguese elite rather than a Castilian elite, imposing the language as part of a unified administration, Castilian would still be in the same general situation as a prosecuted language.
I'm not saying that the fact that Castile had and has more people and more money doesn't help. But it wouldn't help as much as you expect.
The Portuguese would walk around *Spain as if they owned the place, which they would, because their voice and vote would be worth more than a Castilian or a Catalan, telling them that speaking Portuguese is being cultured and not to speak the language of peasants, bogmen and mucketymucks. That's what happened in France historically, that's what happened in Spain. You do not speak proper language, you speak a "dialect", a "patois".
Rather, in a democratic *Spain as a crown union or as a democratic multinational state the more valid argument I see for *Castilian being the basis of a *Spanish language is not because of the amount of people that speak it, but because it occupies something close to the medial point of the Iberian dialect continuum. Once you get past the accent, Catalan (particularly Valencian dialect) and Portuguese are very understandable and rather easy to learn from Castilian.
With a broad standard for *Spanish, the resulting situation would nto exactly be bilingualism, as Portuguese and Catalan would be extremes of the same language. It would be richer, more dynamic and more varied than Spanish (and maybe it would be easier for *Spanish speakers to learn other Romance languages, being exposed to more variation in accents, speech and morphology)