Abolishing national borders is the worst idea ever. How could anyone support it?

Why is it that these things don't see more implementation?

Politics.

Funnily enough, Democrats and Republicans agree with each other on economic policy more than they agree with economists. . . Not to mention, it's pretty easy for high-profile political figures to call in advisers or point to economists who support them despite the fact they are outliers (like Gerald Friedman, for Sanders) and give themselves an air of empirical legitimacy.

Elections aren't won or lost on economic technicals. Although it's worth pointing out that developed countries have indeed made strides; securing the independence of the central bank perhaps being the biggest, despite the fact there is no obvious political advantage to such a move. The implementation of EITC in the US, the support for free trade among the establishment, the move towards cannabis legalisation, taking low wage workers out of income tax altogether in the UK etc.

There's a lot wrong with how we approach policy, but equally the problems we have now are only really there because we have a strong enough foundation to find these specific issues worth debating about. At least we have good enough institutions that we can argue about optimal minimum wage policy or the appropriate tax system, as opposed to, y'know, how to make sure the population has enough food so half of them don't starve.

Well I can't say there's a whole lot I disagree with Trump.  A few, but nothing major except maybe police body cams

Nope, there's only trash here.

has done a better job of convincing me than any of your arguments made for it in the past

I don't know why. Nearly every time this topic comes up, I link academic articles which show investment, wage growth and a bunch of other things we all want to happen would happen if we abolished it. Maybe we haven't discussed it as often as I think we have, but discussing it in a forum context--naturally defined by debate--probably has something to do with it.

however, i'm still curious as to how you could support eliminating the FLSA

Note that the FLSA includes more things than the minimum wage, such as child labour laws. As I said to Flee, the optimal minimum wage is a function of the size of the low-income labour market in a given area. The problem that minimum wage seeks to address is one where monopsony labour markets exist--basically markets where employers have almost all of the power--and thus wages are below the productivity of the given low-income workers. The min-wage is there to ensure that low-income workers' productivity is reflected in their wages. (There are also issues with 'efficiency wages' whereby paying higher wages actually increases productivity and doesn't result in disemployment effects due to reducing turnover, but they aren't that relevant to this conversation).

But, basically, minimum wages are best set locally or at the state-level, given differences in the size of low-income labour markets across metro and state boundaries. What may be an appropriate minimum wage for NYC, for instance, is probably not an appropriate minimum wage for Bumfuck, Mississippi.

But, as I also said to Flee, failing some kind of positive development on minimum wage policy, I would absolutely support a higher federal minimum wage of something like $10/hr. The thing is, historically, states have usually been more important than the federal government when it comes to minimum wage policy. 

and raising the retirement age

Zero percent of economists on IGM's booth disagree with the proposition that pension spending is basically spiralling out of control, and unless restrained will result in either severe austerity, a federal bailout or a default. Auerbach and Gale also find that the fiscal outlook for the US is less than positive, and an ageing population will certainly not help this trend.

The thing is, the state pension (in both the US and the UK) essentially amounts to a pretty massive redistribution from the not-so-well-off young to the more-well-off elderly, and on a personal level I find this to be unjust. On top of that, it's also pretty clear that state pensions will prove to be a massive burden on the government unless something is done to alleviate that. Personally, I support individual, private and heavily regulated pension accounts. But raising the retirement age is not exactly a bad idea given the situation governments are likely to face, and how much longer people are living.

I mean, to give one example, when Lloyd George introduced the state pension in the UK, not a lot of people lived to the age where they could actually claim their pension. Now, almost everybody does.