Are diet sodas harmful?  Probably.  Are regular sugar sodas harmful?
Definitely.  Chances are the diet sodas are no worse and may be a little
less bad than regular sugar sodas.

I have no reaction to aspartame but I do know a small number of people
who do.  It blocks any attempt for my wife to lose and she's gone
through several cycles to demonstrate that happening.  I prefer Diet
Rite or Diet RC Cola over either Diet Coke or Diet Pepsi because of
this.  Diet Rite was the first on the market to switch to Splenda as its
sweetener.

Of course eventually problems will emerge about Splenda use just they do
with all artificial sweeteners, but there's no way they will end up as
common as aspartame problems.  And I know my wife is one of the few who
have some sort of reaction to aspartame and neither of us have short
term issues with Splenda.

I've known several people who drank diet sodas nearly to the exclusion
of regular water.  They are fat.  Diet sodas have no calories so they
may not contribute to weight gain but they certainly do nothing to help
loss.  But is loss really about the liquids as long as you drink actual
water for your daily quota and have sodas in addition?  I don't think so
unless you're experimented with stalls and seen the stalls break as soon
as you stopped aspartame.

The idea that vegan food is healthy is false.  There are valid reasons
for going vegan and if one of those reasons applies to you then do it.
But please don't use a false reason and then end up wondering why yet
another attempt at dieting failed.

Cheaper in terms of time and convenience.  If you compare the dollar
cost of going to fast food places for a month with buying on sale meat
and low price veggies for a month the dollar cost isn't going to be
higher for the healthy food.  And if you use beans in a crockpot for a
fraction of your food that makes it a lot cheaper in dollars.  The cost
of the healthy foods is the effort not the dollars, and in the learning
to buy based on price and the initial shopping trips that you end up
buying the worng foods before you relearn.  The first month may cost
more dollars but not later months.  It's the effort that's higher.

And once you consider the price in effort, making the inherently
problematic vegan approach healthy takes significantly more effort.
Humans are omnivores in objective fact and so it is very easy to cover
our metabolic needs by mixing meat and veggies and fruit and nuts
without much advance planning.  Just have some of each and pay a little
attention to variety and it works.  With vegan it takes planning to
acheive a healthy result.  More price in the coinage of thought and
effort.

I think the more critical aspect is the fact that fast foods are
designed to trigger cravings for more of them.  They tap instinctive
cravings for rich/fatty, sweet/starchy and salty, and they do it with
little other nutrition so they don't really let the hunger go completely
away.