Then what if Zheng He reached Europe then and established trade relations? Would that be beneficial?
Why?
Zheng He had more than enough countries to explore within Southeast and South Asia, and it remains uncertain whether he or one of his subordinates visited East Africa, suggesting that even the latter region was geographically distant to visit. It's also worth noting that the vast majority of the admiral's visits involved trade routes that were firmly established and well-known to the Chinese for decades, if not centuries, although he did expand upon some of them, and mapped out several uncertain routes among well-known regions. He also visited many of them specifically in order to
reconfirm their status as tributaries, as many of them had become "disobedient" from the Chinese point of view (not to mention various pirates causing havoc across numerous coastal regions, which encompassed many of China's tributaries). A significant number of soldiers also accompanied the traders because they needed to protect their trade goods from uninvited pirates and bandits, not because they wanted to directly seize and permanently hold large chunks of land. In other words, visiting Europe would literally have been the last thing in Zheng He's mind (if it had been at all), who continued to travel by sea for almost three decades.
While the voyages themselves initially resulted in significant economic benefits due to the volume of "exotic" goods involved, the court continued to oppose building and sending such a large quantity of ships out to sea, as they would continuously drain the treasury over the long term. The latter was of particular concern because it would make it extremely difficult, if not impossible, to sustain funding for troops along the northern border, not to mention various other domestic projects. The government also decided to tackle pirate raids by imposing the
Haijin, which essentially entailed banning all maritime trade through a large navy, although trading ships along rivers were allowed. While this ban only lasted several decades, as it further incited social instability, and became pointless when the
wokou died down soon after Hideyoshi's nominal unification of Japan, it also proved that the government was willing to go to extraordinary measures on a domestic level just to curb piracy, leaving it with little to spare for overseas expeditions.
Even if Zheng He did manage to visit Europe, the immense geographical distance involved would have forced China and Europe to eventually resort to middlemen. At this point, it would have made more sense to reinstate the Silk Road, as less middlemen would have been involved along overland routes, although the maritime ones would probably still have continued to function. I already explained why it would have been extremely difficult, if not impossible, for China to establish and continuously maintain colonies, mostly due to logistical and financial reasons, so I will not repeat the details. In addition, Europe (beginning with Spain) decided to travel westward because the Ottomans managed to maintain a monopoly on trade between the East and West, due to its status as an influential middleman, which meant that the latter was forced to pay for overpriced goods. On the other hand, while China's tributaries comparatively benefited more from trading, the country had less significantly issues with funding (as opposed to Europe), as it had virtually no middlemen to rely on. China also had no particular reason to trade directly with Europe because it already had enough resources (the latter only had coins to offer for centuries), while numerous European traders were willing to take the risk to extend trading routes further east and west in order to pay much less for the goods in question.
As a result, direct trade between Europe and China doesn't make sense on multiple levels.