Fundamentally disagree. Look at how often people use emoji, AlTerNaTiNg CaSe, and other tricks to convey meaning online (and mainly from their phones). English is not losing complexity, it's not being dumbed down, or becoming simpler because people are typing on a touch screen. It's evolving in real time because the methods of communication are evolving. 300 years ago we capitalized nouns in English for emphasis on important nouns, some people capitalized all nouns like in German, we used punctuation quite differently. Before that, we used these big, chunky symbols to indent our lines and make them more readable (and then we got rid of the symbols, but kept the indents, and that's why we indent the first line of a paragraph). We used to use 3 little stars (an asterism) to break paragraphs apart. Emoji, alternating case, ellipses made of commas, weird punctuation uses, capitalization forgone to convey a flat tone, periods missed at the end of a sentence to give it a friendlier tone, all of these things are living, real grammar rules. They're just not the same ones as in the text books. Much like how AAVE is not taught in schools as correct or proper English, but it has its own grammar rules that are consistent among its users, the way people type now is fairly consistent, and follows a series of rules that have been agreed upon in the same way that any language agrees upon its rules.

English got frozen in time because of the printing press, and we all just assumed it had been the way it was for so long that that's the way it was, English was set. But languages are constantly evolving, and there's really no such thing as a correct way to write a living language. There is only the way we currently write it, which is constantly changing. Normally that change is fairly slow, but we with virtually every other aspect of humanity, the computer, the Internet, the smartphone, and other rapid technological changes increases the speed at which we adopt new cultural norms exponentially.

It's also worth noting that we write more now that we ever have before. In the past, you may occasionally write letters, jot something down on a postcard or a birthday card, have a weekly shopping list, or a note from a phone call, but apart from secretaries taking short hand, stenographers, and other specialized careers involving writing specifically, the average person did not write anywhere near as much as the average millennial or Gen z, who may communicate exclusively through text, email, discord, whatever, with their friends on a daily basis. With more writing, it follows that the language will evolve faster. A short lived species will often demonstrate more mutation and variation than a longer lived species. There are just more generations to work with. Every email, every Facebook post, every text message is a potential for a random mutation in the language, and without the strict confines of previous generations telling us that something must be written in a certain way, because the primary purpose of writing in the past was much more professional and academic, much less casual, those mutations are given a chance to survive and to flourish.

Yeah, sometimes it is genuinely hard for me to read posts made by younger Gen z or Gen alpha, and it is especially hard to read posts that are trying to mimic the gen z/alpha posts but from older generations, but I am legitimately excited to see how English evolves without the confines or academic interference. I can almost imagine a high/low English split coming, and that's exciting.

I try, it's just sometimes I get excited

wi kan't ivən get piipəl tö spell kərrektlii wið þe lettərz wi hav.

kapitalayzeyšən, and pⱥnctueyšən ar biyiƞ lost bükoz i'c slaytli inkonvinyənt on ə fown kiiybord tu inklud ðəm. eniþüƞ awtsayd ov lowərkeys a-z iz tuu hard naw. göd lⱥk gettiƞ eniwⱥn tu šüft ðe kiiybord tö faynd ⱥðəə karakterz.

No? If anything, the frequent typing of text on phone keyboards in online interactions has created even more intricacies to language; as now we are used to not only using the proper methods of capitalisation and punctuation marks to convey meaning, but also to intentionally leaving them out or distorting them to convey more meaning or emotion. Phone keyboards can auto correct, auto capitalise and add punctuation to sentences for you. People go out of their way to disable those and intentionally type "incorrect" sentences because "On my way!" vs "on my way"/"omw" just don't feel the same.

Also, not bothering to type "proper" sentences while texting your friends on a phone keyboard doesnt mean you start to just forget that sentences begin with a capitalised letter and end with a dot lol. Furthermore English is just a stupid language with stupid spelling and pronunciation rules. The concept of the spelling bee was really shocking to me when I first heard about it as a non-native speaker becuase most other languages just... are easier with spelling.

edit: some people really are just basically illiterate though (see what I did there? no capitalisation.)