No there definitely should be a backup base, or several. An entire information empire playing every major power in the galaxy against each other for the benefit of one individual and he doesn't think to preserve it beyond "durr I hope no one scans this planet!" or worse the admittedly unlikely possibility of actually getting tracked down? What would've happened if Liara didn't care about Feron and just wanted revenge, and instead of an on-foot infiltration she just had Shepard Thanix the **** out of that ship? Bye-bye Shadow Broker? Without any sort of backup and recovery?
Please.
In the end, a backup facility doesn't mean much if the Shadow Broker himself is dead. Let's say, in this scenario, that Liara did have Shepard simply have the ship on Hagalaz bombed from orbit, just to be sure. What then? I'm loath to use the phrase "because plot", but I think that would apply here. In this story, it would be more than likely that if there were backup facilities, we'd simply know the locations of the backups, because of Cerberus' intel, or the intel we gathered for Liara on Illium. We'd just pick off the Shadow Broker bases one at a time throughout the galaxy, but that's not as interesting as actually encountering the Broker himself. In this case, also, the Shadow Broker's resources would simply be inconsequential to the overarching plot of the trilogy, since it would be disposed of.
Well I'm glad their opinion mattered more than anyone else's. The suggested NPCs were admittedly not sound considerations from a squad composition point of view but that doesn't mean any proposed substitution is invalid and "just an excuse to get rid of her".
Now why the squad composition argument can be handwaved is because any character not previously in the squad can be given just about any powers you want (with some limitations). Feron for example could be an adept (and no I don't care about the comics). I don't personally think Feron's a good idea either but there are other characters that could easily fit the adept bill, if that's what you're concerned about.
Oh how about the fact that Garrus and Tali don't get replacements if they die? Does someone then ****** "oh noes there's no engineer or infiltrator(soldier?)"? OK EDI's an engineer but if you don't have Ashley (and you don't for a good chunk of the game anyway) you're potentially left without a sniper. Oh the horrors. In that case we just have to deal with it.
This is simply the pattern I've observed. I won't say that this is absolutely the case for everyone, but whenever the subject of replacing Liara comes up, it's more often than not framed around the general dislike of her character.
As for other characters that can fit the bill, which ones are these, and how? The problem with most of the characters Shepard encounters is that they are all disposable. Now, if we want to rewrite certain points in the entire trilogy in which some characters are fixed points that are immune to death, then that's a different story, but by ME3, just about everyone can be dead, save for at least one ME2 crew member required to help Shepard at the end of the suicide mission. The fact that this last squad member can be just about anyone makes it problematic. The only real substitute for Liara would have to be a totally new character, like some adept Aliance marine added in like James Vega. But since Liara exists no matter what, there's not much point to that.