The gemara you're referencing is part of a series on Shabbos 140b about Rav Chisda giving sound economic advice.
For example: since bundles of vegetables & sticks all cost the same, Rav Chisda suggests that one should buy a long bundle to get more bang for their buck.
In the middle of Rav Chisda's list of economic advice, Rav Papa chimes in with some seemingly economic advice of his own, saying
"if someone could drink beer and drinks wine, he transgresses Bal Tashchis."
To which the gemara rejects this, stating that it is more important for one to not to harm their body (ie drink wine) than save a few dollars (by drinking beer).
As you pointed out, The Maharsha's commentary seems to suggest that Rav Papa's opinion was due to his own benefit or personal bias.
Responding to the Maharsha
The Rashash (1794-1872) is quoted in this article as explaining the Maharsha's comment on Rav Papa as follows:
ואני אומר על פירושו (של המהרש"א) ולאו מילתא היא דמאי יענה על ר' חסדא (מאי נוגע בדבר היה) דהאי "ולאו מילתא היא" שב גם כן על דבריו, ואולי דלזה כוון ר' פפא לעיל (קי"ח ב') באומרו לדידי חשדן ולא הוה בי, כי יש שהיה חושדין אותו כפירוש המהרש"א
And I say on (the Maharsha's) commentary that it's not really anything, because what is Rav Papa answering to Rav Chisda (ie what incentive did Rav Papa have in this) and this "לאו מילתא היא" can also apply on his words, and perhaps this was Rav Papa's intention earlier (Shabbos 118b) where he says 'they suspected me and there was nothing of it,' because there were those who were suspicious of him, as the Maharsha writes.
הרש"ש מסביר, שאולי דבריו של רב פפא על כך שחשדו בו לשוא, מוסבים על הסבר המהרש"א לדבריו בנוגע לשכר, שאמר זאת כדי שיקנו ממנו שכר
The Rashash explains that perhaps the talk about suspicion of Rav Papa was in vain, explaining the understanding of the Maharashah's with regards to things dealing with beer, that Rav Papa said this so people would buy beer from him.
My understanding of this is that the Maharsh"a is saying that Rav Papa WASN'T giving this advice for his own benefit; however, there were those who were sometimes a little suspicious of Rav Papa and his business dealings (see Rav Papa's Wikipedia entry for background and also this kipa.co.il article) and if the topic was about beer-- even if Rav Papa was giving genuine advice!-- these people would suspect him of having an ulterior motive.