I don't think signing top players while they are at top clubs is the problem. Like a few posters said before, it rarely happens anyway.
The issue and the question is, do we stand a chance to sign Neymar from Santos, Hazard from Lille, Lewandowski from Dortmund, Griezmann from Atletico, De Bruyne from Wolfsburg etc. So basically players who attract the attention from all the clubs.
I don't think we're appealing to young players currently. I believe we will be forced to risk a lot of money with transfer targets like we did with Martial until we are on top again.
But with this there is often a special story.
The Lewandowski transfer was already done when any of the big clubs got interest - already 2 years prior to the time he transferred. Then he would not have been the bargain. What happened... United bought Kagawa and Lewy was not allowed to leave. The next season it seemed he already had a vocal agreement with Watzke that he could leave - he just could not prove it (there was some story around it). And then Bayern got Götze because of the clause with the fixed transfer sum. That totally destroyed everything. I guess Lewy never wanted to play second fiddle to CR7 and maybe others or go into a rivalry with Benzema - that would not have been good for his brand and he and his agency always looked for that, too - so they could not destroy that bargain.
Bayern never had spanish players and Martinez came prior to the spanish wave now. He told that he liked Bayern since 1999 - he went for the loosers of that final. His girlfriend already was taking spanish classes and had relatives in Germany. And - the most important - Heynckes started to phone him and chat with him almost weekly already half a year prior to the transfer. The spanish players really seem to love it in Munich - not just the club but I have seldom seen professionals and their wifes that really tour the city and area so much - and the cafes and restaurants - and post so many pictures of mountains, lakes etc. - it is like a holiday prospect. So I guess in the future - even without Guardiola - Bayern will not have any problems getting spanish players.
Since Lizarazu - and then Sagnol - it is no problem either to get players from France to Bayern. They really loved the time in Munich and as Liza is commentator on French TV and Sagnol was coach for the U23 national team they spread it around. They already were a big reason why Ribery came to Germany.
Vidal... Vidal came to Leverkusen as an unknown player. The scout for Leverkusen then - Michael Reschke. Bayern wanted him in 2011 - everything was already done and dusted - but Leverkusen did not let him go - and Vidal did not have the patience to wait until his contract ran out (and Bayern then was not as desireable as it is now). Reschke works for Bayern now - and when he knew Vidal was somehow available, it was not a problem anymore.
Same source - and probably extra knowledge through Vidal - was used for the Coman transfer. Bayern was on him before but was not lucky - they got to know that he is unlucky in Turin and like this made the deal. Interesting is that a lot of players from Chile now think that playing in Germany is interesting (I guess with Chicharito we will see the Mexicans next)
Costa - like many many more he was scouted since years by Bayern and watched - but he first went to Shaktar and then Shaktar called for a too big transfer sum and did not go into contract dealings to clubs who did not want to spend so big. Last winter - during the Ukraine crisis - some players including Costa - went on strike and did not return after their winter holidays for some days. At the end they did. But that caused Shaktar to let Costa leave for a lot less than they always wanted.
Just some little stories about recent transfers. And maybe to show that additional criterias count or that all of the transfers have a special story.
I do not know if Munich e.g. is less attractive than London is. If you are not the sun freak it is about the best city to live in - Munich even more than Turin or Milano who have a worker image, too. Culturally top - less than an hour to big lakes, an hour into the mountains, an hour to Austria, two hours to Italy - and about 3 or 4 to the italian beaches, to Verona and Venice.