I was deeply beholden to MT4 years ago because MQL4 language is like a skeletal version of C and relatively easy to code. In contrast, MQL5 language is more like C++ and carries is a steep learning curve. As Metaquotes continues to update MT5, it is less and less like MT4. MT5 now has exponentially faster communication with trade servers, supports trading via Python scripts, and incorporates standard code functions for machine learning.
The retail forex market has a very high turnover rate which means that there is always a fresh crop of new traders, and they likely prefer MT4 for its simplicity. Given the steep learning curve of MT5, I suspect that broker-dealers will continue to offer MT4 as long as the demand is there--even if Metaquotes permanently stops updating MT4.
I've noticed many broker-dealers tend to promote their in-house platforms over MT4/5. Hey, if they can get a new trader beholden to their in-house platform, the trader might be more reluctant to switch broker-dealers. They also promote the hell out of mobile trading apps which is rather unethical. This is a thinly veiled attempt to gain more customers (who might not even be able to afford a desktop machine) regardless of the inherent communication delays in mobile data/Wi-Fi connections.
I would recommend trading MT4 as long as you can if that's what you're familiar with but at the same time, download a free demo of MT5 and start getting familiar with it as a "hedge."
“[A]s we know, there are known knowns—there are things we know we know. We also know there are known unknowns—that is to say we know there are some things we do not know. But there are also unknown unknowns—the ones we don’t know we don’t know.”—Donald Rumsfeld, 2002