Hello @Ramos ! After reading your topic/post I'll point out my personal opinion:
I think it's fine as it's now, I don't see any problem regarding this matter.
Just clear your way to the bank before entering the bank by arresting/killing the criminals protecting the bank-rob from outside of bank, it's simple. Snipe them from distance or I don't know, I don't find it hard or impossible to do! DE can assist you too if there's 1 or more CLO members presented at the bank-rob or at outside of bank.
Or just get good/better as @HasomY said, or just don't go to bank-robs if you don't like this or you're too lazy/tired to clear the bank's area from dangerous criminals helping/protecting the bank-rob (as @Rzz0 said). No one forces you to assist bank-robs.
Also, you're going to a bank/rob, what do you expect? To don't find any difficulties/complications? To have it easy? Of course you'll have to work your ass (alone or with other cops/DE members) in order to stop the criminals, break their defence lines and their bank-rob. Criminals have their tactics, yes, but cops have them too! Just think and plan good what you're going to do. If there's a presented defence line at outside of the bank that it's being robbed formed by... 10 criminals (for example) that are camping there... call for backup, think and find a way to rekt them.
"Just let the cops enter a bank atleast...": Why criminals should do this? Don't see logic in this (from a criminal point of view, lol).