No language is too structurally difficult to become a lingua franca. Languages become widely used due to the language becoming economically or politically necessary or through becoming culturally significant.
In fact, though Mandarin's character system and tones are difficult to learn, Mandarin has several features that make it much easier to learn compared to European languages. Mandarin does not have subject/verb agreement, plurals, conjugations, or tenses.
People do not have to fluently read and write Mandarin for it to be widely spoken People around the world do not fluently read and write English because they just use English as is necessary for their purposes, and English is still widely used.
Professor at the Flinders University College of Humanities Jeffrey Gil stated, "The inconsistencies and irregularities of English’s writing system show that linguistic properties alone do not determine whether a language becomes global. I conclude that a character-based writing system will not prevent Chinese attaining global language status."
Thanks to technological advances, written Mandarin is no longer a barrier to widespread Mandarin communication. Handwritten Mandarin is no longer a basic skill since smartphones and computers can generate characters from Pinyin Romanization or speech-to-text. Many Chinese people just use smartphone technology to communicate in written Mandarin.